Great post! Pretty much all that is said about the user interface also applies to the ~contemporary Autodesk Animator for DOS (open source here: https://github.com/AnimatorPro). It has the advantage of running in DOSBox.

Not having layers is frustrating, but also in some way fun. I have not used Deluxe Paint much, but in Animator there is a second Clip screen, it is possible to save and load the clipboard (i.e. "CELs") as well as the current image using keyboard short-cuts, and then there are some other nifty features like "copy everything that changed in this frame since I came here"... The more I play with it the more workarounds I find for things that otherwise would have been easy to do if there were layers.

Also have that set up on my phone, with a lot of on-screen buttons configured in DOSBox. Works well and doesn't come with any of the annoying in-app-purchases and/or ads that all the app painting programs seem to be full of. I even bought a stylus to use with Animator on my phone.