None of us know what kind of offers Grant is going to get but I think it is overly simplistic to say he is an 8 pt / 5 reb guy. Grant is a decent player with a decent track record, including some big playoff games. Players like PJ Tucker get paid (perhaps overpaid). Grant may end up one of those types of players. Whether it is $8.6M (the QO) or $10M, or $15M, signing Grant is going to put us over the second apron, unless we do some salary mitigation such as trading Brogdon for a pick.
To me, the Porzingis trade which added about $7M, pretty much locked in that we are going to be over the second apron, or so close to it that we can't do anything to improve the team anyway. That trade counted in 2022-23 season so the new CBA rules don't apply to it. We could add salary. I think if anything, the Porzingis trade is an indication that the Celtics have accepted being over the apron and the decision on Grant will depend on what offer they need to match, what other player are available, all the normal things.
It is true that we don't want Grant on a "tough to trade" contract. I don't know what that threshold is, but I am sure it is part of the calculation