There are multiple ways to fix this [Onimaru’s bA → D&C loop] that wouldn’t affect other uses of these moves:
increase pushback on blocked bA just a little bit (perhaps increase it only if the opponent is cornered to not mess up bA to B spacings at long range?),
make bA not cancellable to D&C, leaving only cancels to Tactical Slice possible, not unlike Argagarg’s jA which can only cancel into Sparkling Bubble;
decrease hit/blockstop on bA; this keeps the loop but requires Onimaru to guess whether the opponent blocks or not instead of letting him react to it, resulting in some interactivity,
heck, you could probably add a specific version of D&C dealing one (as opposed to two) chip damage that only triggers if bA lands; Rook already has this in form of C throw after Thunderclap.
Bumping for relevance, since as of the latest update there is now a character that has zero counter-play to these loops, namely Quince due to his nerfed DtQ.
You could possibly make the second hit not a knockdown on standing opponents but juggles when they’re airborne, like Valerie’s jA. You can still get the reward for guard crush, but the opponent has the opportunity to escape. Give it a very small frame difference for recovery on hit, something like + or - 1.
I like the idea of D&C only doing 1 block damage, but instead of taking out the first one, I would make it work like Lum’s jump B where the first hitbox that gets blocked does 1 block damage and the rest do 0.