Right now my albums basically are in random order. That is because Roon, say, will order 3 Mozart pieces differently. Some under M, some under the name of the music, some under the name of the conductor. Is there a way to group all Mozart pieces together?
You can search for Mozart in Composers. Still a bit messy for unidentified stuff though, I’m afraid.
Roon tags are the best way. Select a bunch of albums and then give them a Roon Tag. The Tags appear in searches, which is cool.
I use Roon Tags to get over the inability to display by folder. I give each folder content it’s own tag. It’s a PIA work around, but it works 
(P.S. This also will allow you to search for non-identifiable stuff)
Make sure that Mozart is credited in Roon as the Composer and you should then find the various works listed in Composer view under Mozart.
How do I do that?
This KB Section will give you the details. Basically you go to the three dot menu when viewing the Album and choose Edit Album:
Then go to Edit Album tab and choose Edit Credits:
Add the Artist and choose Composer as the role:
You can edit a number of albums at a time to add the same credit to them all. See under Bulk Editing in the linked KB Page above.