1001-1003 had plenty of good action being performed from both sides. Your interpretation of the Yonko not doing anything since then isn't a fact, Oda is just the master of off-screening stuff.
If anything it's implied in the latest chapter that the action has continued since we left off, but since the SN haven't posed a super serious threat yet, and they have taken the initiative so far, BM and Kaido have basically just repelled attacks and enjoyed seeing their tenacity.
The fact that they still have their cocky smiles pretty much shows us that the Yonko haven't needed to go all out yet.