Nextcloud it just too heavy I totally agree, and everything feels slow and sluggish.

For just files I use Syncthing and couldn't be happier, it just works in the background without a central server just syncs the files between phones, PCs and laptops by itself. I set it up like 5 years ago when I had enough of Nextcloud and to be honest most of the time I forget that I have it, but I use it every day to sync my password database for KeePassXC, my music, my private and work documents between all my devices.