I’ll also try not to poke a hornets nest here too!!!
Successful Zwift racers have big 1 min and 5 min power, and most can sprint. Endurance isn’t so much of a factor as you’re right that most races are short. (that said, if you do want longer races, they are there)
You’ll improve at racing on Zwift, by racing on Zwift. There’s an art to playing the game, which you can’t learn by training. You have to do it. Learn how to stay in the draft and recover when you can, learn when the effort will ramp up and when it will ease off. Climbs on Zwift are never easy. Even the short ones have a particular method to them. I’ll never forget the time I figured out how to ride the short climb on the London Classique course - strangely satisfying.
If you need 20 min sweetspot efforts, ride a race on the Innsbruck course or Epic KOM. You need 2 min efforts, Watopia Forward KOM, etc etc…
I’d suggest racing 2-3x per week, pay attention to your recovery and don’t bother with zone 1/2 rides. Focus on your top end. Join a team. Learn to “play” Zwift rather than just focusing on your power. If you haven’t already done, register on Zwiftpower and ignore the “official” Zwift results at the end of a race - the Zwiftpower results are the ones that matter.
Hope that helps?
Oh, and join a team!!!