Hi there.
My thoughts were like this:
The Vero 4K is always on, as is the NAS.
The Vero is connected to my AV receiver.
Why not play my music with it?
But - there is always a but - I want to control it via my tablet or smartphone which is also always on, not via the TV which is almost always off.
On my PC I use musicbee…
Is there a similar addon for kodi with proper capabilities for handling my music library and playback control options?
And a proper remote app that I can connect to it?
Use the built-in webapp or Yatse
Yatse (Yatse™ Best Original Kodi Remote for Android) is even more than that
Alternatively use Kore Kore - The new official Android remote | News
I think I tried yatse once with my raspi…
But yatse and kore are just remotes, aren’t they?
I was somehow looking for a musicbee-like GUI on the TV and tablet (whichever I choose).
Both have a similar GUI to Musicbee. They are free so perhaps try them out.
They are remotes that display your music library (artwork) on your tablet
Do they?
I admit I only tried the movie interface once…
I’ll look into them.
Where to find Infos how to connect to the Vero?
Any preferred method to do so?
I would for a start mount my music folder via fstab?
You just activate the webinterface under Settings → Services → Control and give it a username/password for protection
And then you give the IP/Username/Password in the App and it connects 
Well not related to your other question but best way to use your NAS is by mounting via fstab.
Wow, yatse found the Vero when starting up without me doing anything!
Did not try to use it, but it shows my movies.
A bit scary - do I need to look into my security settings??
But apart from that, thanks so far, I’ll test it out and come back with questions… 
Yeah webinterface is anabled by default but as it only reachable from you LAN (unless you connect your device directly to the internet or have portforwarding) not a security issue but personally I have configured username/password (which you can do as described above).