I guess you should look into nextcloud and/or synology drive, because they do exactly that.

You can use it as a “cloud”, where the files you are using are downloaded locally and then updated if you made changes, or you can just sync everything.

For me, i have some folders i need always available, internet or no internet, and some that i don’t care about as much. And that’s how it’s configured, it syncs the stuff i need, it leaves the stuff i don’t.