Once upon a time, I had a Motorola A920 in the early '00s that came with a dock to put a second battery into to charge it (pre-CGM era of course), but something like that would be lovely when combined with a foldable or flip phone.
After Google pushed a firmware update to my Pixel 6a today, it definitely gives a feeling of vulnerability. https://support.google.com/pixelphone/answer/16340779?hl=en
Would love to recommend the Fairphone, but not available in every country. One of my cousins wanted one (his dad is diabetic too), but unfortunately not available to buy in Canada.
Hopefully Framework humour the idea of a phone at some point, but it'll be difficult to please everybody, and I'm not sure it's economically viable.
The Samsung XCover are mil-spec with swappable batteries (major selling point on their site) and don't struggle with being waterproof, so it can be done. I don't think it's as niche an idea as people would expect.
I excitedly followed the Ara for years! Was gutted it never amounted to anything.