I checked out your edit and what's going on is pretty weird. my recommendation (if you don't wanna just use the native import/export feature and give up on DB dumps) is to (on the new server) create a minimal tandoor docker-compose.yml, run it, do minimal setup, see if it works, and if the fresh container works, check the permissions, then just replicate those perms for your other postgres directory that contains data you actually care about. good luck!
yup, this is what I do and it works great. just be careful if you add plugins that store a ton of metadata that changes a lot (or logs) because that can lead to some sync conflicts and you might wanna exclude them. I've just been ignoring the sync conflicts tho and haven't had issues yet so maybe you could do the same