I have one of the original Sig M400 pistols with the pistol buffer tube, which came with the Gen 1 rubber slide-on pistol brace. When the concerns from ATF arose, I removed it and discarded it. What is the most popular legal replacement for this buffer tube/SigBrace?

This is not exactly an answer. But the pistol brace rule is currently 'stayed' so any 'replacement' is in a legal grey area with respect to its long term legality. But I've used the SB Tactical SBA3 for awhile and its certainly workable.

When the tyrants tell you to give up your arms is the time to be preparing to use them. Not disassembling them.

and that has exactly what to do with the brace rule situation? Not needed in this section...