Samsung XCover 5 seems the best bet atm, to the point I'm surprised I hadn't heard of it earlier; smaller than just about any other android phone I've encounter

Samsung XCover 5 seems the best bet atm, to the point I'm surprised I hadn't heard of it earlier; smaller than just about any other android phone I've encountered in years (almost iPhone Mini sized), has a removeable battery and theoretically should have zero need for a case. The double combo of Samsung promising longer support cycles and it technically being an enterprise model mean it should be supported longer than most Android phones would be too. New they're a bit pricey for the specs but you can find them like new used on ebay for half RRP.