Firstly just enjoy riding. There are many here who are targeting moving towards becoming elite athletes. Hence their FTP is a reasonable, not perfect, measure of their progress and comparison to their peer competitors.
For the rest of us, the FTP is relative to oneself, eg, over the last 3 months is it better than before, ie, a bit higher than your last test, 77; 5% improvements are considered large. The actual specific number is less important. FTP per kilogram is useful for you because your body is very used to carrying around 320lbs, so it already has inherent strength, far greater than my 143lbs, so as you lose weight, so long as you keep exercising, your FTP/kg will improve, and you will get faster in every way, on your bike, walking, etc.
As @OreoCookie has already said very well, a MTB hardtail is probably your best bet until your weight is below 220lbs, ie, most road bikes have a weight limit about that level. So, don’t spend a huge amount on the MTB bike, as it is only a step on the path to a new you. I’d consider getting a 2nd hand Hardtail.
The upright body position on a MTB will mean 90% of your weight will be on your saddle and pedals. Whereas a road bike shares more of the load over your pedals and handlebars, which at 320lbs is likely to cause significant discomfort to your upper body from hands to your shoulders, neck and back. Even on the MTB you may need a different saddle.
Your indoor Peleton, will probably gets lots of use long term. As when winter comes, and you get fitter you’ll start to do more exacting workouts, eg, Intervals, and they are best done in a more controlled environment, eg, indoors.
But, your MTB will be what you use for enjoyment, the fresh air, scenery, maybe with a friend, passers by, nipping to the shops or cafe. While doing that also getting fitter.