There are a few nice hints in the comments already, but I'd like to remind one feature that may actually be "the dumbphone" experience but more modern.

Ultra power saving mode on Android.

It may differ from vendor to vendor, but in my experience it works exactly like that (been able to use that in old Samsung, Xiaomi, Realme, Moto). It changes screen to greymode, simplifies launcher by a lot, cuts notifications, still allows to use a selection of apps, including gps.

It customizable - you can keep some color, choose which apps can be accessed (on my current phone up to 6), make calls, text or IM. As a bonus it takes a few days to drain the battery.