Could drill holes in the rock, glue rebar in and fit the post into 2-3 rebar prongs.

Or drill and bolt post bases into the rock (or make your own 90 degree angles for ancoring).

Or a beast of an auger that can handle rock? For crumbly rock I’ve used jackhammers for planting trees with good success

Have wanted to do this approach some day. https://youtu.be/YBhgge17vw4?si=9T5FdtUZ5qGOefBI