While I agree there is something nice about using a folder paradigm to organise content — indeed a folder structure is still the bedrock of archive practice in museums — almost everything that can be achieved with ‘folders’ can also be achieved with tags or bookmarks in Roon.
A tag approach will also offer more flexibility as an album (when using folders) can only (without the use of aliases) live in a single parent folder, but an album (when using tags) can have many parent tags.
Start out by creating a number of tags to represent the top level folders you want ie. 'Mellow Listening", “Sunny Day Listening”, “Albums to Revisit”, “Old Favourites”, “Christmas” etc.
If you want to mimic ‘nested folders’ then create a new tag for the child ‘folder’ and then add it to an existing parent ‘folder’ ie. create a tag called “Carols” and add it to an exiting tag called ‘Christmas’.
You can then simply add your albums to any level, simply by adding tags to them.
If you have already spent time organising your collection using folders on a file system there are several posts on the forums on how best to migrate that structure into Roon.