I used the Nokia 8110 with KaiOS as a digital detox phone during holidays. It has working email, whats app, google map, hotspot. It has enough features to "survive" and answer or triage the occasional fires from work. And it is unusable enough that i don't use it for non important stuff (chatting, browsing, etc). Ironically its bad UI/UX is it's main selling point.
(At least I'm down to only using one phone at a time. That's progress, right?)