And Android takes like a full minute to boot back up.

Yeah, I miss my brick Nokia's quick startup time... I think there's a way to avoid the shutdown problem.

1. Hibernation mechanism, known from PCs, could be ported to Android. This way you can pick up where you left off before swapping the battery. Seriously, I hate losing all unread notifications after a reboot. Who the hell though that's a good idea?
2. Hot swapping battery, which means that you can change one half of the battery, then the second half and the phone won't shut down at all. Foldables make it easier since they already use 2 batteries, 1 for each half. Just wire them up in parallel and the voltage won't drop when one is taken out for replacement by the user.