I think you have answered your own question pretty much S K, what you are doing in terms of compound moves is already the best for a really strong core, if you dont currently do any barbell squats they are great as well. I used to do cable crunches and decline weighted crunches which added resistance to crunch movements but honestly, I wouldn't bother anymore, I have stronger abs than ever and I dont train them specifically.
One exercise that I did find interesting for a while was a leg raise and windshield wiper superset on the floor whilst holding a bodyweight barbell in the extended arms benchpress position. Can be fun to do and works the abs pretty hard but then again, I have stopped doing that as well.
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