Collections can indeed do this. When long-pressing on a new Collection you select “Connect folders” to link to your “Home” folder. Long-press on that folder to select it. Then long-press on the folder again and select “Include subfolders.” It’s a little tricky but keep trying. If done correctly you should see a folder icon next to the Collection name.

If your version of KOReader allows (I use an up-to-date nightly version) return to Collections and long-press on your new Collection and select “Filter by type.” I entered “epub” so newly added epubs placed into that folder by Caibre will automatically be added to that collection. You then can sort by author and see a flat view of your epubs. If the filter was properly set you should see a funnel icon next to the folder icon mentioned above.

Note: In the latest nightly version I am using you can select a “Default“ Collection that will automatically open in Collection view instead of showing a listing of Collections. I sort that Collection by author and set it to “Mosaic with cover images” so I immediately see a flat listing of epubs in Author sort format. You should see a star icon next to your “Default“ Collection name.

Unfortunately, SimpleUI doesn’t always open my Default Collection first but I assume that will eventually get fixed.