You could trade Crowder and keep Marcus if you are worried about tax dollars
No you can't. Maxing Thomas alone puts Boston into the tax and trading Crowder to keep Smart won't save dollars it will actually cost more as Smart will get a contract that exceeds 10 million. This is the exact type of thing I've been talking about for awhile. If you keep the win now players, you will lose the players that could be solid pieces around the win later players. KO is already gone. Smart will be next and that will only continue in the future. These are the types of things that happen when you try to have your cake and eat it to, and that says nothing for getting the young guys valuable playing time or maximizing their window by creating more assets.
If the real goal is to win a title with Brown, Tatum, BKN 18, LAL, etc. then Boston should be creating more assets for that group and allowing them to play as many minutes as they can play so they can work through mistakes and what not. I believe Hayward would fit in well with that group, with his age, talent, and position, but Horford certainly doesn't and I don't think Thomas will either as I expect him to age much faster than Hayward. Crowder is going to leave when his contract is up as well, so there is no reason not to trade him now while his value is at its peak. Again, that is if the real goal is to win a title around the young guys.
On the other hand, if the real goal is to try and win a title now with Thomas, Horford, and Hayward, then Boston should absolutely be trading some of the assets now to acquire that player or players that can get Boston into the true contender status. Unfortunately, Butler and George, two players that could do that, just got traded, so I'm not sure where that next player is coming from and thus I believe Ainge missed his window for going this route. So he should really be looking to the young core and doing what he can to maximize their window (which is not wasting assets or stunting their development).