Personal opinion of a random dude on the internet is I would go sustained power build every time. Anaerobic conditioning can be done relatively quickly. Improvements to endurance and sustained power take longer and I think are more applicable to your cancelled Fondos. For real world riding and zwift, it never hurts to be able to sustain more power.
Even for Zwift races where the selection occurs on a short 1-2 min climb, I rather have a higher FTP than well developed anaerobic system. The anaerobic condition helps in the climb and maybe a little after and possibly with repeatability. The higher FTP helps with the lead-in, as you’ll be further under threshold, it helps on the climb as you’ll be less over threshold, and it helps after as you’ll be further under threshold and can recover better.
My power profile is not flat and I am much better at 5-20+min efforts. If we were the same wkg, did a race with a big climb and got to the end together, you’d have a much better chance of beating me than the other way around. Just depends on your perspective, either you are equally weak or equally strong across the board. I’d lean towards the latter view.
One caution I would mention about zwift racing is to pay attention to your body and don’t over do it. I dug myself into a hole a couple of times prior to embracing the structure of TR with too much zwift racing. Its a lot of fun and I found that I would sometimes go too deep and do too many races and build up too much fatigue. But it is also a lot of fun and a zwift race was where I learned for the first time what it was like to really go deep.