Tried 3/4 of the tools, and none helped me reattach neovim.

Ended up using dtach. Needs to be run ahead of time, but very direct and minimal stdin/stdout piping tool that's worked great with everything I've thrown at it. https://github.com/crigler/dtach

have you tried diss, shpool or abduco?

also vmux appears to be specifically tailored to vim/neovim

https://github.com/yazgoo/vmux