I just added this to the original post.
to be more transparent.
Monday - off
tues - recovery/spin legs for 1hr
Weds - Intensity for 2hrs. Vo2, Threshold, etc.
Thurs - 2~3hr endurance
Friday - active recovery
Sat - 4-6hrs long ride with Threshold efforts, or VO2 efforts. Rest of the ride is endurance.
Sunday - 2~4hr endurance
In my training the bigger question is how does the endurance work impact my interval days. So if you think adding power (why 5W?) and “see how it felt” is more about the impact on next 3-5 days then yeah, pedaling 2 hours around 56-60% FTP can be low if the focus is performance, or if doing 25 hours a week it might be just fine.
I’d ignore heart rate and ride endurance based on RPE. Sometimes if I want to turn my brain off I’ll set a .65 or .70 power target and ride ERG on the trainer for my endurance workout. I will take note of heart rate, but unless it’s wildly high I won’t let it change my workout.
OP,
This may sound out there… but do you do any running? If so when you go for normal “training runs” (the equivalent of endurance in cycling) how does your heart rate compare?
I too am one who has to push harder to get my heart rate up for “endurance pace” on the bike. It’s not where my legs are falling off, it is just that the easy stuff feels way too easy.
I average about 6-8 hours/week, ftp around 260 right now. FWIW my easy endurance rides start at 60-70% FTP, and progress up to 65-75% FTP.
With your 230W, for an endurance ride I’d do 15 minute warmup to get out of town, then 30 minutes around the middle of 60-70% (138-161W for 230ftp) which would be roughly 150W and then ratcheting up to 165-ish for 45 minutes, then 15 minutes around 175W (75%) and then 15 minute cooldown. But I’m a fan of progressing power during endurance work. If feeling fatigue I’d ride a little longer at 150-ish and progress more slowly and skip the last 15 minutes at 75%. But again thats me.
I’m not saying do 5 more watts because the power is “low” in an absolute sense. I’m saying add a bit more power (and 5 watts is my default “baby steps” amount) because the OP said the power felt too low and there was significant negative decoupling on the ride.
Unless we’re saying that significant negative decoupling is a non-issue. 
just above posted what I would do, I’d push the first 30 minutes to around 65% which would bump it from 130-ish to 150-ish. But again thats what I would do, after having a top-tier coach for 3 years and going thru another top-tier coaches Winter group training program (Tim Cusick’s JoinBaseCamp).
Here is my EASY PEASY 2 hour endurance (based on BaseCamp)
and my more performance oriented 2 hour endurance (adapted from my coach):
Always let the power come to me, and then push power.
The default middle ground is what I posted in words above.
I don’t run often, but when I do, it’s usually when Im traveling without my bike. HR is usually at maybe 160bpm, which is quite high! I run for maybe 40min, just so I don’t hurt myself running so much out of the blue. 160bpm HR feels very comfortable to me. But the effort my legs have to put out in order to get my heart there is not comfortable.
will give this a try and see how well I recover before my next hard day.
Sounds like you’ve got a bunch of ideas on what to do and how to go about assessing how it went. Have at it! 
So I’ve got a similar max HR (I’ll see high 190s a couple times a year) and I never try to see 140Hr on an endurance ride. If I did I would certainly be adding more fatigue than necessary.
An example endurance ride I did recently was NP 197W (~325W FTP) and average HR of 121BPM.
How’s your nutrition? And do you have this same issue if you were to do an endurance ride that didn’t follow an intense workout day?
To me, high leg RPE during endurance after threshold/VO2 sounds like maybe you’re not eating enough carbs/calories between these rides.
My max HR is also 200. I only do z2 by HR and keep around 120-140bpm. It’s just a range! Some days it’s easy, some days it’s slightly harder. Just stay within the range to avoid future fatigue.
Z2 should be a pace that’s sustainable for multiple hours with only moderate fueling. If it’s not, your training in the wrong zone or should train more muscular endurance.
My nutrition on the bike is great, easy to measure, easy to eat a lot. Off the bike…not so sure. I try my best to eat a lot but sometimes I just can’t eat anymore.
My endurance rides after a rest week is very different than the same rides within a training block. I can probably ride at 65-70% of my FTP and feel fine. I think to expect the same level of effort during a training block is probably silly, and will do more harm than good. Endurance rides after an intense workout day is one of my least favorite things just because even 56% of FTP feels bad (soreness at first, but loosen up eventually as the ride goes on).
FWIW at 8-11 hours/week, during the loading weeks I’ve been able to sustain multiple days per week at 66-79% on the 2-2.5 hour endurance rides (or portions of interval days). But those are pretty fatiguing blocks of training, and after 3-6 months I need a short 1-2 week break to reset and let go of long-term fatigue. Almost thirty years older with stressful job but kids have been gone for six years.
Individualize for yourself, do what you need to do for you! Seriously if 56% ftp feels hard after an intense workout day, don’t think twice about keeping it easy and remember the focus is on conditioning and long-term volume. The long-term adaptations, both cardio and metabolic (leg), are driven by low-intensity muscle contractions so you are highly unlikely to be missing out on anything other than stressing yourself out thinking you have to always be pushing power. Of course if you are highly focused on performance at 12 hours/week you might want to consider eventually ratcheting up power but never at the expense of your intense workout days.
Ignore heart rate, you’re worrying over nothing, your power range is fine. 
Just ignore HR. Especially if you are doing the work after hard days.
1 thing you could try is to switch your Tuesday and Wednesday workouts. I bet Thursday endurance will feel better and may have higher HRs.
This is normal and the same for me. If you’re on the trainer in Erg, switch to resistance mode. Then ride and gradually increase power as your legs warm up. Depending on the day and previous workout/fatigue, this can take anywhere from 10-30 minutes. It’s not uncommon for me to start my endurance rides and low z2 feels like a struggle. Only to continue my warmup and eventually high z2 feels good. Use RPE and adjust z2 based off of how you feel. If it feels like too much work it probably is, so just adjust accordingly.
“Tell-tale sign of being the right place is that you can still exercise whilst breathing through your nose or talking in complete sentences”
I can’t believe I had to scroll this far for this!
Zone 2 is conversational pace, just try talking to yourself and seeing if you can speak in mostly complete sentences without gasping for breath you aren’t too high. It is harder to find the lower end, but I assume most of us want to be mid to high zone 2 so we are getting more work done - which I’m sure is probably not what is best.
I would try to get up to the point where speaking in mostly full sentences gets to be difficult and scale it back slightly. I will also start a little harder for a few seconds to get the HR heading in the right direction.
Fueling and recovery might be part of it so just try to focus on improving in those areas if you can.
You ride more than I do, especially right now, are you just a little under recovered, could you add some more liquid calories around rides?
The running thing - it doesn’t apply to you but can be a common experience where doing higher end cardio on the bike was really hard at first because my legs would die way before I was at what my HR should be based on running so that would have been a possible explanation (just really good cardio already and legs that aren’t used to cycling).
I think the answer here is, pick one metric - power, HR, or RPE, and use it for your endurance rides. Don’t try and make all three match, often, they won’t. Each of them has it’s own issues that skew them, and while on some days they might all be in sync, on other days some will be off for whatever reason.
Personally I’d just ride “endurance” by RPE and don’t worry where power and HR fall (maybe use them as upper limiters to avoid going to hard, but not as lower limits.) Endurance is quite a wide zone, and where exactly in it your power or HR sits for a given workout is far less important than doing the workout in the first place.
I’m curious what RLGL is telling you. :D.
I think your “z2” rides are too easy, but given a 6 hour threshold/vo2 ride the day before I could see why you might not want to ride z2 for 2 - 4 hours.
my 
take is if you can’t recover by your next workout you might be doing too much. And by recover I mean complete the ‘intended’ workout. I’d say the ‘z2’ you are talking about is probably ~160w and the HR is probably ~ 140bpm. Just as you suggested.
But other people have their takes as well. Ultimately the best action is to listen to what your body is telling you. :D. If it feels too easy it might be, and if it feels too hard…it might be. And if you know that it feels harder than it should, you’re probably on to something. 
