Interesting stuff - kinda jealous you have the time for such fun. Anyway, I think I answered your OP question with the eBay link. I’ve never built one myself - and I definitely can’t point you to which are better - but at least you know they exist, and can hopefully find your way from there. Good luck!

I’ve just had my first bank go puffy (RAV Power, I think?) not all that long ago, and it looked like there were 18650s used inside already. I just haven’t gotten to trying to replace them yet, so I’m not sure. Have you looked inside yours? I don’t see why you can’t just exchange them if so. Might require a minor bit of de/soldering work.

I remember seeing charger bank shells on eBay.

Finally, I think a full blown RPi is serious overkill for your intended purpose. At least consider a Pico - much smaller, cheaper, and actually intended for simpler uses like this (although it’s still kinda overkill).

I do find your hot-swap idea interesting, why would you need to do this if you had pass-through?

Sealed batteries = forced obsolescence. Bet you can find a way to replace the battery if you really want to.