I’m a moderately heavyweight Diskstation user since we mostly run my wife’s business with them. What we use on a DS1815+ & DS412+ (after much time trying packages to figure out what is / isn’t good):
File Station (occasional web access to file shares)
HyperBackup (backup to / from a secondary older Diskstation)
Download Station (web interface for managing large downloads (HTTP(s), torrents, etc)
WebDav server (because some folks needed it to reach files remotely)
Plex (serving video)
Video Station (serving videos to devices that can’t use Plex)
Snapshot replication (hourly BTRFS snapshots of live volumes for rapid recovery of files (short of Hyperbackup restores))
Docker (to host additional services that don’t have native packages)
Homebridge server + plugins (to add non-native stuff to my household Homekit)
Things we don’t use (or stopped using) because they are bad solutions (for us):
Antivirus Essential (never did anything useful & slowed things down. Tested over a year before chucked it)
Central Management System (overkill for 2 servers)
Cloud Station Server / Sync (overlaps w and less reliable and worse supported than iCloud which we already use)
iTunes Server (used to use it, but it is ancient, deprecated, and only barely works. I really wish there was a modern, working, supported version!)
Photo Station & Audio Station (overlaps w and less reliable and worse supported than iCloud which we already use)
Comments & Discussion:
We were actually running more, but some services have moved to an expensive router in the last year instead
Synology sometimes changes strategy / direction over a few years & this can create potentially confusing overlap between solutions (especially with backup technology). for example: Do i want to snapshot files, replicate shares, replicate an entire server, setup a fail-over server, setup a nightly backup? Those can all do parts of the same job, but they are distinct solutions that are best suited to solving particular problems.
The GUI makes the tools easy to use, as you’ve seen. But that doesn’t mean it’s easy to understand them & the documentation is often lacking. It helps to have an IT background.