After a restless day, I finally set up my #guix system with the root filesystem on tmpfs!
I've documented the process in my dotfiles repository: https://codeberg.org/look/misako/src/branch/main/install/README.md
This has been on my to-do list for quite some time, I hope my steps can help others who are looking to do the same.
Big thanks to @Z572 and @anemofilia for some file-system ideas.