I really don't think Didi is essential, and even a three-year deal would be regrettable (much as Cutch is only going to provide around one full year of value on his). Sign JT and you still haven't actually raised payroll (he makes $10 million now and if you give him $24 million Didi's own salary covers the rest)
Then the only question is do you QO Didi? Not doing that would also be regrettable, as you want the pick, and you'd also want him on a one-year deal.
If he leaves, sign a platoon shortstop, sign a platoon second baseman, and you can get plenty of production out of what you already have while better addressing the pitching. New GM will have a lot more options next off-season (including big-name free agents like Lindor, though he'll probably get traded). And if they win, there's the deadline.
There's zero chance they are going to exceed the threshold, the bigger concern now is how much lower will actual payroll be. But continuing to just add payroll and contracts isn't going to be make the team better long-term. Not for us, not for Harper.
I think you still have to sign JT. If a new President comes in and decides there has to be a teardown, it's not like he can't be traded. I think they can also make major trades, they just aren't going to be easy to do, and we certainly can't be trading from the minor league system. But they might not be interested in winning a bidding war for JT, if Steve Cohen wants him badly enough (his net worth is 14 billion; Middleton's 3 billion. Silly to say he can more easily afford to spend his personal money when it's all pocket change to these guys, but so it is).
Essentially we are back to the 2015 from office structure. The fact that Gillick was president and Amaro was a lame duck didn't stop them from making moves. Nobody thought Amaro was actually the one making them. MacPhail has been a GM three times, he and Rice should be perfectly capable of doing things, and already familiar with the long-term depth chart projections and salary obligations.