I'm a Wemby skeptic (compared to Luka as prospect for example), but even if it was Luka I wouldn't operate my franchise in that way. I'd put it under some consideration, of course, but not enough for me to take a worse trade for Donovan for example. It's 14% if and only if you're the top team (most of the tankiest teams you admire have failed to get the best odds), landing the #1 pick doesn't mean Wembanyana is automatically better than whoever is next year, and landing the generational prospect is not the same thing as winning the title.
I don't believe you get the best deal out of Donovan if you put the deadline on trading him before the season starts, so I wouldn't do it personally. You're absolutely right that timing is important, but that's not something I'd try to predict. You could just as easily luck out next year. You can't just look at the success and say "man that would have been if they had waited". You would also have to look at all the high draft picks that were taken that leave a team saying "man it would have been great to have had that pick a year later". This dance is not something I'd rely on and even if you're 100% right about Wemby, you're talking yourself into a very unlikely scenario to begin with.
It would be a stronger argument if you believe the top 5 were better than the following year's top 5...I think that holds some weight. But that can always backfire too. The draft is a real crapshoot, and I'd prioritize the best package for Don over tanking immediately.