IMO, the Celtics are targeting Beal, whether it's now or next summer, and they're not going to sign anyone who might interfere with that.
Fournier is gone unless he comes at an extraordinary value.
There are two way to think on things. One is to retain players so that you can trade them [for Beal if you want] and still retain players on your team. The other side is that if you go after Beal in free-agency, you have to be prepared to be able to either dump people without getting salary back AND renounce the rights to just about every free-agent you have (and TPE and Exceptions)... that's with Horford still on, and Beal probably taking a pay-cut.
If you let go of Horford (unable to dump him), and assuming we don't reach the Finals before then, then you still need to pay him $14.5M, but now we're in better range depending on what Beal agrees to and even then probably would need a slight pay-cut.
So you can potentially have Beal-Brown-Tatum and no one else (probably a straggler here and there), or maybe even Horford if Beal takes a substantial paycut on his first year... and pretty much an empty roster including no 1st round picks, etc.
Maybe it'll be worth it, but it's a huge gamble. I'd rather trade for him if possible, if there's actual interest in him here.
Those are the simplified scenarios we may be looking at.
You also want your team competing, in these years else Brown and Tatum won't be quite happy around the franchise... but bringing big names always alleviates that (if they end up working).
Or we could do a Sign & Trade during free-agency, which kinda brings us back to the first scenario... and Washington agreeing as well.