Going to have to try that remote rebuild, because right now my 10yo T420 takes around an hour to rebuild gitbutler-cli (not yet packaged in NixOS), so that'd be real nice.
I know syncthing has a solid reputation, but I'm a bit reluctant to try it on my home directory (it's super mission critical). Does that interact well with home-manager+SOPS?
Mostly, when I'm away from my workstation, I just connect in from my personal macbook via the "wezterm tmux" feature, since most of what I do is either web or terminal. The need for a "pick up and go" copy is nice but not really necessary. Though in the event my main workstation fails, I should check to see how long an rsync from the restic backup would take. If it takes the better part of a day, that might be compelling to use syncthing, especially as my workstation is a 6+yo laptop.
That's how my devices are configured, and it's amazing - if I need to travel for work, I just pick up my laptop and go. NixOS makes sure the system itself is identical (apps, most of the code configs, etc), then any app-spcific config is handled by Syncthing. It was really magical the first few times I used my laptop on the road!
Also, if you aren't already across it, you should look into remote builds/deployments. Basically, the regular nixos-rebuild command supports deploying over SSH. It's nice because you can do the heavy work (nix evaluation, building non-cached packages, etc) on your grunty workstation, then have them pushed across to the laptop via SSH. Then, when you want to apply that build to your PC, it'll be super fast (since everything is already evaluated & built!).