You could just add a symbolic link of your home directory into a seafile library. Seafile does not ignore symbolic links. Be careful not to have your seafile directory inside your home folder, as it will cause a loop (seafile directory inside home directory -> home directory linked inside seafile directory), this is the main reason sync of symbolic links is disabled in most sync software.
Usually on linux systems there’s a
.cache directory inside home directories, you will most likely not want to sync this, add an exception with seafile-ignore.txt, as it will waste space on the server unless you disable library history and run garbage collection frequently. Do this for each cache and temp directory inside the home directory. The reason those files are still on the server (and why seafile’s sync algorithm is miles ahead of nextcloud’s) has to do with seafile’s architecture being based on content tracking, git implements a conceptually similar but more simpler system as it’s designed to handle text files, each library being analogous to a repository in git terms.