It is true that you are free to change the config on a VM manually. I included it because it's the only other service I could think of where you might set up SS

It is true that you are free to change the config on a VM manually. I included it because it's the only other service I could think of where you might set up SSH authentication. This is interesting about GitLab, thanks for the pointer. As you mentioned it only appears to be configurable instance-wide, there is no way for a user or organization to set it up for themselves (for example for your gitlab.com organization). That makes it a bad replacement for SyncThing'd private keys as moviuro suggested.