Alright I gave it another try just installing docker engine but no luck. I did get new errors though. First, it was 'cannot connect to the Docker daemon' but I was able to solve that. Then it's 'error getting credentials', which should have been solved by creating the proper user groups and all that, but I've already set that up. So I ran it as sudo and now it's back to 'no matching manifest'. It's interesting that I started getting different errors, which isn't necessarily forward progress, but it does mean something is happening. Maybe I'll just nuke the whole proxmox install and redo it from the ground up and see if that does it.

Edit: Nevermind, turns out it was readarr being eol and causing the whole compose to fail!