Millsap is the one potentially interesting one on the list but he is an undersized PF who is 36 and has been on a steady decline in terms of production and minutes since he left ATL after the 2016-17 season. Although likely an upgrade over Grant Williams, he is not worth being hard capped for.
I believe that the Schroder transaction was made official on 14 Aug 2021. It is posted as a transaction on CBS Sports, ESPN, Real GM, and Yahoo Sports. I didn't check any further. I know it has not been announced so maybe it is to some extent pending, I don't know. In any case, I don't believe that the Celtics are going to do a deal to hard cap themselves at this point. It seems Stevens made that clear in a recent interview and that may be why they went ahead and finalized the Schroder deal.