> technically my flip phone, a Kyocera Dura, is an android device, but it can't install apps so I'll consider it suitably "dumb"

You can actually still sideload app stores (F-Droid, Aurora, etc.) on the Kyocera Dura. I bought mine specifically so I could still put my company-mandated pager app on it. Turns out I was able to get a physical pager instead, so I never had to go the sideloading route.