I thought by copying the contents to the Nextcloud data directory directly (via rsync) then sync the folder would make it faster, but I was mistaken.
Copying the folder to the Nextcloud server (running on Pi 4) took only roughly 10 hours, but syncing via Nextcloud will take me +/-6 days!
Rsync and nextcloud don’t use the same protocols.
One doesn’t use SQL.
The bottleneck is your raspberry pi.
But even with a good server you won’t get faster than rsync.
What you can do is do a rsync for having the folders inside the pi. Then put those folders in your nextcloud user data folder (/whereyournextclouddatais/myuser/files) then perform a chown on those folders :
chown -R www-data:www-data /thepath
After that perform a scan :
sudo -u www-data php -f /var/www/nextcloud/occ files:scan myuser
After that Nextcloud have all your TB in your user folder.
Then next syncs will be quicker because you will just modify some little files.
Totally forgot about the database component. Thanks!