There is really no scenario where we are going to be under the cap which means that we have no "room" to sign any free agents through a conventional signing (other than resigning our own such as Fournier). We have the MLE and we can do sign and trades. Sign and Trades could involve Celtics players or use of a TPE but sign and trades result in us being hard capped (as many have pointed out) so we are limited. but there are tiers of things we can do.
Our biggest need is a PF (or a swing who can legitimately play PF). If we are going to do any kind of a sign and trade for an upper'ish tier FA, to me it should be for a PF. Markannen, Collins, someone like that. The salary for these types of players will exceed any TPEs we have (the biggest is $11M) so this is going to require sending out Smart and Thompson and possibly a young player or pick. That is giving up a lot plus ending up hard capped. If we are going to do something like this, it better be for a really good player who fills a major need.
The next tier is to S&T for someone who could fit in a TPE (up to $11M) with potentially picks added (or young players, finagled as separate deals). That does not buy you all that much these days on the NBA FA market, not a starting PF, and likely not a starter at all. I am not sure there is going to be a value for us via this path. Is there a good enough bench player available to spend this kind of money and assets on? Maybe a starting PG and move Smart to the bench (probably not Lowry)? I don't think so but I am sure I have not thought of all the possibilities.
The last or lowest tier is to use the MLE. The non-tax payer version of this is $9.3M but using this would in a sense hard cap us also. The tax payer version of this is $5.9M That is enough to get a decent depth player and I suspect that we will deploy this, probably the tax payer level to keep our options open. Maybe a back up PG? Maybe a bench big?