Starter won't engage flywheel

[Thicc.s13]

Ka24e freshly rebuilt and dropped in won't crank, the starter just spins and sounds like an rc car when I turn the key. I've tried just about everything including new battery, new starter, checked all grounds, new blue starter relay, cleaned all connections. Solenoid reads about 6 volts and I read it should be 12v, so I believe its an issue with that but I can't for the life of me figure out what it is. I also tested the old starter (after buying the new one bc I'm an idiot) and it worked fine, so that rules out the starter I would think.
Edit: tried hooking the little plug connected to the solenoid straight to the battery and it just makes the starter spin like it was before. Could this mean its bypassing the solenoid somehow?

UPDATE: I was able to make the starter solenoid "engage" by wiring it straight to the battery, but not spin at the same time. When I go to turn over the car I can hear it engage at least partly but it seems to not go all the way or something.

Other info: Headlights also stopped working, they're hooked straight to the battery with a fuse (intact) and switch in between. The car has a fuel pump kill switch in between the fuel pump and ignition switch, not sure if this would change anything.