I wrote a bit on why Nextcloud a while back, I'll link it here (see point 5 in FAQ): https://memories.gallery/faq/#faq
As such, Nextcloud doesn't really lock you in; it just provides a framework for the app. You can, theoretically, continue to use Syncthing to sync files while running Nextcloud on top of it (probably not ideal though)
I want to note though, the "no lock-in" philosophy refers more to being able to move out of Nextcloud/Memories at any point if you want. Nextcloud still just stores everything on your disk as folders and files, so you can just decide to nuke it one day and still have everything (not fully true yet, you'll still lose some things like tags and albums; exporting these out too is WIP)