>>76789519
make it harder instead of going for the isometric equivalent of high volume.
ad:
https://www.youtube.com/watch?v=peZOX8ORbQE
or move smoothly through side plank - plank - other side plank
it makes sense to have some reluctance to experiment with exercises, because you can easily discover a dangerous variation, but you look for prior work. (While writing this post I wondered if pressing a weight forward with the free hand would make for additional core work, but the shoulder hated that idea.)
the Progressive Calisthenics Manual has a side lever progression that starts with four side planks and then goes into dragon flags