I have some machines in the garage that I've made up electric panels for. Those panels are screwed to plywood squares that, previously, have been able to be screwed directly into drywall/studs. This new building has a block wall and I was wondering how to mount these panels.

I've heard of various concrete block fasteners but they all seem to rely on 1) the block being filled, and 2) the block not cracking when drilled. Not a chance I want to take

So instead I thought I should just screw a 2x4 to the wall's top plate (pic) and have the electric panel attached to that 2x4 (or with the plywood between them). Can things be screwed into the top plate? Either from the side as planned or from the top with very short screws. What should I avoid?

You can screw into the top plate, it's fine.
You're unlikely to crack the block while drilling. Use sleeve anchors or shoot nails into it.
Or glue if you're feeling wacky.