I don't know much about trainers or Zwift. I doubt I ever will.
Since we are both randonneurs, you may appreciate a perspective that I wouldn't normally note. When I was custom building a randonneuring upright, one of the key factors was control or minimization of vibration because it is my belief (supported by a few studies) that vibration to the human tissue isn't merely an efficiency matter (as Balor has explained many times) but also a significant source of human fatigue. This is why I went with a magnesium frame, larger tires, a leather saddle, long wheelbase, lots of rake, latex tubes, and one of those fancy Specialized seatposts. That bike was flat out comfortable for very long distances. When laying down on a recumbent seat and riding "normal" country roads around here, the level of vibration into the body is much higher. So, how does this relate to power output? If we compare the performance of an upright to a recumbent, it really is not so simple as a point estimate or a single short test loop. Horses for courses. Actual comparisons via controlled studies have been done looking at power levels, I think I have read all the relevant literature; I have done my own comparisons; and, there really isn't any need for me to see any new data. If I were to link the studies and if anyone cared, they would find some reason to not accept the results. I should note that I never seen a study showing MORE power in a supine/recumbent position versus upright whether submaximal or maximal power. Besides, people believe what they want to believe. I think Psychs call it confirmation bias. I know I gave it a full 2 years of my life, so, mine are now set.
For my first 6 months on a bent, my FTP about a measily 150 and peak power was 550 watts. I eventually got the FTP back to my upright personal best of 286 watts but my peak is over 300 watts below that of the upright. I took my upright out the other day for tests on a hill that I use for intervals, my first upright ride in 2 years. I blew away all my power levels from past upright days and well ahead of my recumbent power levels. If I had not done that one ride, I would have falsely concluded that after two years, my recumbent performance matches my upright performance.
If recumbents have so many advantages over uprights and few disadvantages, why are there so few of us? I could probably list 20 reasons very quickly. I'd guess 3% of riders at PBP were on a 2 wheeled recumbent bike. I rarely see one on roads that I ride or I should say I never ever see one in the hills, once in awhile on the flat river road. I ride on very popular roads and usually see more than a hundred uprights during weekend rides. There is a very large recumbent bike store only 15 miles away. Access isn't the issue. What is it?