(I’m not an iTunes fan, please!) but iTunes does this very well. I have a root directory with all the big subdivisions I am interested in. What do I want to hear at the moment: Rock? Jazz? Fusion (my definition thereof)? Folk? Classical? and so on. Note all these can be changed deleted added at any point. If I click on Rock I see all my albums plus a 2nd tier of the hierarchy: Beatles, Stones, Clapton, Dead, etc. (some would argue Dead should elsewhere, which is exactly why this should be manually curated. You do it your way, I do it mine.) Click on the Dead, and I see all my Dead albums. For Rock, almost always I want to hear (or select from) an entire album, and for some albums I will want to edit out certain tracks. Easy with playlists.
OK I picked Rock on purpose, because I really want to talk about Classical. It works for both. For Classical, the 2nd tier can be period or composer or both, of course. You will probably want the greats up there (Mozart, Bach, Beethoven…) but if you have dozens or more of Baroque composers you probably want them in their own separate folder, or folder and subfolders, organized by what makes the most sense to you. At the bottom of the Classical hierarchy, depending on the piece and the album I organize by composition, or performer, or album. I can think about what I want to hear at a high level, and then get to a specific performance at the bottom level, all very quickly. I don’t have 10k albums, but I think it would work well for a large collection, unless you need to look at 1k albums before you pick the one you want. Maybe you need to browse 50 albums (or pieces). And note, once you put in the work to organize something it should never go away, never get automatically “corrected” (read, messed up).
OK, how to implement in Roon UI? I think conceptually it is simple. In the playlists display, there’s all the room in the world, indeed it seems rather bare to me. So put the hierarchical list on the left side (expanding and collapsing as you click through playlist folders), and at each level of the hierarchy just display that subset of albums (to the right of the playlist display). Once you get to a playlist or set of playlists at the bottom, then display those tracks. Allow searching, focus, etc within subsets of playlists to make it even more powerful. Import hierarchical playlists from iTunes. Expose the playlist hierarchy when you want to add a new albums to your lists. Of course, allow you to edit your playlists, folders, etc. And don’t ever automatically change the contents; only the user gets to do that. Manually curated hierarchical playlists.