First of all an overview of the Celtics salary structure for next season (currently based on the salary cap staying at $109.1M and luxury tax at $132.6M). Blue: player options, Green: team options, Red: renounced Wanamaker, Black: dead cap, Pink: minimum deals Waters/Fall, Yellow: rookie scale on projected picks.
With Yabusele, Jackson as dead cap and assuming Wanamaker is leaving, that's 19 (potential) players directly in the mix for 15 roster spots. After reducing this to 15 players (making some decisions on the lower end of the roster and picks) the Celtics will always be well over $140M in salary and thus in the luxury tax => repeater tax.
Trading some small contracts or stashing picks can save a million here and there, but that's not going to make a big difference. If Kanter leaves (not exercising player option/traded) that's not enough and picking up the team option of Theis for the Celtics is basically a no-brainer to not leave a huge hole at the center spot (and even without Kanter and Theis
it creates very little wiggle room for the Celtics to operate).
The only ways for the Celtics to get under that luxury tax is trading one of our core guys (Walker, Hayward, Brown, Smart or Tatum) and take less salary back or get an agreement with Hayward for an extension on a lower annual salary. I think the majority of us want the latter and it should be feasible. Pre-crisis I estimated that Hayward could possibly get a new contract on the open market at about 15-25 million a year (his max days are over). Right now I'd think it would be on the lower end of that.
For the Celtics it could be interesting to opt for a 3-year deal and thus having Walker and Hayward expiring at the same time to create cap space in the summer of 2023 (although with all this uncertainty in the world it remains to be seen what that's worth with Tatum (upcoming max) and Brown on large contracts).
The salary number for Hayward next year needs to drop to at least $28M or less. At around $23M the Celtics can afford to keep Kanter and fill out the roster without much problems (while still having to make decisions what to do with their picks and players like Waters, Langford, Edwards, Green, Ojeleye, G.Williams, R.Williams, Poirier and Fall). In that 25-28 million range it's (probably) necessary to trade Kanter (and Poirier).
So ideally Gordon Hayward signs a new contract for 70/3 ($23.3 million a year). Which would be 36 million for 2 extra years added to his player option of 34 million. Although Hayward's agent will probably think getting more than 18 million a year on the open market in 2021 is easily achievable, it's still guaranteed money right now.
In the current NBA climate it's hard to say what players will accept. Personally I think Hayward would only accept something in the 25-28 million a year range and that Kanter needs to go to avoid the repeater tax. I assume ownership wanting to avoid repeater tax is a priority for them, if not this post is a complete waste of time