Hello,
(If this feature already exists, please forgive me, I have searched, but couldn't find it.)
TL;DR:
My suggestion is fairly simple (conceptually at least): When you choose "Gallery" as image source, you can either choose one folder or all folders. I think it would make things so much easier if you could select a folder and all it's subfolders. It gives you easy and complete control over which photo's to use in a slideshow and which to ignore without having to use a filter. It also makes it easy to have multiple slideshows (you can have a folder 'Slideshows' with subfolders 'Slideshow A', 'Slideshow B' etc). Perhaps you could make launcher shortcuts with the source folder as a parameter, so you could easily start different slideshows without have to change the settings?
My use case: I have an Android TV box with a large monitor that I use as a media hub/dashboard (Spotify, Netflix, slideshows, Home Assistant). I have a pretty large number of pictures (~25000) on my NAS server. These are synced with folders on the TV box (I used to connect directly to the NAS, but I found that the huge number of writes to the eMMC of the TV box drastically decreases the lifespan, so I switched to syncing). I could of course put all the picture in one folder, but it makes it a lot harder to keep things organized, and also both Windows and Android don't seem to handle these numbers of files in one folder very well (Windows would take forever because it would be too busy generating 25000 thumbnails). Filtering by date is not very useful for my use case. I also have a large number of animated GIFs on the device, so if I select all folders, those get picked up too, but as static images. I tried using filter feature to ignore those, but it doens't seem to be working.