My phone's modem died and I didn't want to deal with all the trouble of replacing the phone, so I decided to go for the more intersting option of going for the PDA model: a flip phone which can hotspot and my old phone for apps. I'm going to keep track of a few things over the course of the experiment. Right now I'm using what's basically a pure feature phone, though I'm waiting for a KaiOS one to arrive now. The one I have at the second (unfortunately) runs Androiid-ish. It's an AOSP base with a fully custom UI and apps. The OS seems to be baked into ROM, there are no downloadable apps, so all you get is what it comes with. Essentially just phone, sms/mms, a camera (a whole 2 megapixels!) a calendar, a clock, notes, and a calculator.

Update: 27/6/22
I've had my KaiOS phone since last week, though I haven't had a chance to dive in too much. I used it for podcasts on Friday, and I unsuccessfully tried to set up a dev environment a bit before that too. On Friday I thought that it had no multitasking capability at all, but I was actually mistaken there. This is the phone's layout:

The "hang up" button when in an app acts like a home button in appearance. When I was using podcasts, music, etc. I would hit that to go to the home screen and the playback would stop. What it actually does is close the currently running app. You have to hit an app shortcut button (the star or the envelope,) or hit back until you see the home screen again. Not the best UX, but at least I figured it out. The more concerning part is that KaiOS launched a breaking new version last year so there are <100 apps by my guess work on it. Additionally, there is no known jailbreak or sideloading method for any KaiOS phones running version 3. Additionally, the one way to sideload apps, which is to register a development account and temporarily upload it to the KaiOS store, is broken and results in a login loop. Hopefully that'll be fixed and/or better solutions will be available soon.

15/7/22
So yeah, KaiOS sucks pretty bad. Its multitasking is fundamentally unreliable, there are no applications to speak of, and there seems to be no real hope of decent application support ever to the point where even the community consensus is "Yeah KaiOS is dead. It's a shame it's not picked up by a competent company." I'd say don't pick it up for any use case but the very specific one I did (just a flip phone to tether to a smart phone,) but honestly I think the battery life on the "android" version of this phone is better so it's not even particularly good at that.