It is very easy to setup synchronization of your device running /e/ OS and your own Nextcloud instance - just install the F-droid Store (which offers only vetted verified FOSS apps) and then install from it the DAVx5 and NextCloud apps. DAVx5 will allow you to sync calendar events, tasks and contacts to and from your NextCloud instance. The NextCloud app will allow you to enable automatic sharing and/or backups of folders (especially useful for backing up any photos taken on your /e/ device) as well as browsing files on your NextCloud server.
Best regards,
Steve Berson