I'd pay Betts more than Harper and Machado, but there is a limit on how high I'd go.
It's a hard choice to make. To me, it comes down to this: if we think we will reset and then pay Betts whatever it takes to keep him, then fine. I'm not so sure we can feel confident in that option and also end up paying him way more than we really wanted to but do it just because we committed to this choice.
The flip side is dealing him to get something worthwhile in return instead of getting a lousy comp pick, instead. We shouldn't just give him away, but how little are we prepared to settle on?
The third option may be the best of all. Trade Betts for the best we can get, even if it doesn't seem like enough. Trade JBJ, too. (Maybe Price & Eovaldi.) Use the money to sign under-the-radar FAs that will help the team as much or more in 2021 and beyond than 2020. Reset the tax, then go hard after Betts for 2021 without going too far overboard. This way, if we lose out on him, we still have something to show for losing him, and we can use the money dedicated to bringing Betts back to sign other good FAs. This plan has one noticeable drawback: we'll likely suck in 2020.
I'd hate to watch a Betts-less Sox team in 2020, but I'm thinking this might be the best option, at this point.
BTW, I don't get what "Mookie downgrade" means.