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.