Seafile has an sshfs style client for windows, mac and Linux. Rather than a traditional folder sync like Dropbox (which seafile also has), seadrive mounts a remote connection to your library that you can browse in your file explorer. I've only used the windows version, it has little cloud icons that show the files are not local and then you can right click a folder of file and "make available locally" to have offline access. This sounds exactly like that you are looking for. Full gui access to all files with no local storage needed unless you want.
I haven't tried seadrive on Linux but they have the option on their site. I use the standard seafile-client on Linux and choose only certain libraries to use with no issues. On windows the seadrive is quite impressive in regard to how well it works.
https://help.seafile.com/drive_client/drive_client_for_linux/