Ok, couple of comments on this. Zone 2 and threshold/SST should not be “either or”. Even within a SweetSpot plan (be it TR based or of your own design), you should have generous amounts of Zone 2 riding (~.65 IF w/ no big swings in VI). There are no strict percentages, despite what the internet says.
Reading what you wrote above suggests to me that you are thinking “Zone 2” and “Zone 5” and “SweetSpot” are mutually exclusive. You have to find a way to blend them all that works for you.
Also, avoid the sweet spot vs. polarized debate ← I just saved you a few years of confusion and/or time waste. You’re welcome. 
YOU ARE ASKING GREAT QUESTIONS! But there is not a simple “go here” to get a definitive answer. All that stuff you learned from Inigo San Millan interview and mitochondria, etc. All valid. But riding in one training zone isn’t going to give you that. Unfortunately it’s not that simple.
Most of what you commented on regarding Andy Coggan is not correct, but it’s not that important for what you’re trying to do.
If I were you I would get an online coach. When you’ve spent a fair bit of time in the TR bubble it can lead to polarized thinking, over-simplification, and a lack of understanding about how and why you’re doing what you’re doing. The podcast, blog, and forum certainly help, but it’s a big time investment. If that’s fun for you, by all means, do it. You’ll be spending a few years closing your knowledge gap. Alternatively, you could short-cut the process a bit and hire someone for about a year. Many ppl on here have a coach AND use TR platform. Nothing wrong with that.