I've got a really bad physical asset and I'm not sure where to start fixing it

All of the tutorials I can find seem to be for humanoid characters and I’m not sure how effective they’ll be

Click on “Constraints” and take a look on the “Angular Limits”. Almost all should set at least to limited. Swing Motion 2 could be locked most time. You have to test a bit how it looks best. Some limits might be bigger (45°)… some should less (5°) some should be locked.

Managed to fix a lot, how do I stop parts from separating from the body and stop parts from spazzing out?

Disable all linear limits. If you have some scaled bones (that are not 1.0 at export) it could make issues at the physical asset as well.