So, good news on the SD Album mode, worked out all the difficult stuff. It is now updating very smoothly.
Every time the Wallpaper is hidden, e.g. because you open a new app or switch off the screen, it updates in the background, choosing new random images from the sd album to show, which takes a second or two.
Then as soon as the Wallpaper is shown once more, it immediately updates the pictures. This is istantaneous most of the time, but sometimes there is a brief delay before it switches, only noticeable when the screen is turned on after a long period of inactivity. I should be able to fix this, just by not updating in that situation, only when the phone is "warmed up", so to speak, so the update will then always be instantaneous. What I want is for the user to simply not notice anything is happening, they will just notice that they are seeing new pictures all of the time.
Happy to report also that the automatic cropping I am doing to turn the rectangular images into squares is working very well, there seems no need for any complex Face Detection algorithms. So that will save processing and battery life.
Now just working on removing duplicates and sorting out the interface so it is super easy to choose the SD card album. Something like this:
I think another week should do it, then will be ready. Might also try to fit a few new frame styles into the release.