So, a few days ago I decided that my life was too simple, devoid of massive headaches, and to remedy that I was going to start collecting and tagging classical music.
Haha, that's a good attitude and mindset to go and kill some classical monsters.
The smartest thing I can probably say about this is that the best system for handling and maintaining Classical music, is your own system.
While I do some things a bit differently then you, it looks like you come prepared, and as long as you decide on a system and stay consistent with it, your system should work pretty well.
My system handles Album Artists, Titles, and the different work levels a bit different from yours, but just copying my system without thinking it through (and having lived through the obstacles that made it as it is now) is probably not the best idea.
What I would suggest: install portable versions of both MusicBee and Picard and create a test library containing some more complicated classical composition. (containing several levels of works)
Some that are present in MusicBrainz database, and a couple that are not.
Some operas, some concertos, some large works from Bach and Wagner, and certainly a 'The Very Best of Classical Music Hits for You' compilation album. (you said you wanted headaches...)
Then do lots of testing on that before deciding on your 'final' system.
Your question about Genre's and Periods; if you use the Classical Extras plugin for Picard, it has some specific features and settings for both under the 'Genres etc.' tab.
You could also install an additional portable MB/Picard setup, and configure them exactly as I described here:https://getmusicbee.com/forum/index.php?topic=28999.0
Just follow the steps and don't give them a second thought.
Then load some releases through Picard, and see the results using those virtual tags in that MusicBee installation.
The 'standard' tags shouldn't differ much a 'normal' installation, but the (single) custom tag and the virtual tags should give very nice results and possibilities.
You might like it, and if not, it could be at least useful for you to see if 'your' system gives some similar results, and if you perhaps will want to tweak your system a bit more in some areas.
One of the virtual tags (which one depends on how much info I want here) that are being referred to in that guide would have to be copied over to <title> though, to jive with the rest of my library.
If I recall correctly how I've set it up: if it doesn't get a match of a Classical multi-level work, it will retrieve the standard _recordingtitle tag, which I think is what you would normally get also.
In case you have a mixed classical/non-classical library and you would want to use the virtual tags for non-classical music too, it shouldn't be too hard to add something like "$If genre category is not classical, A, else B".
F.w.i.w., a while back I decided to try using two separate MusicBees for Classical and Non-classical.
I must say that for now I am very happy with that decision, and will probably keep it that way.
It saves me lots and lots of time and headaches in trying to get formulas, tags and layouts working perfectly for both very different genres.