GB wrote:
Google suggests that you can update Android way beyond v6, using
unofficial updates. Might be worth a look.

https://en.wikipedia.org/wiki/LineageOS

The S4 goes up to Android 11:
https://wiki.lineageos.org/devices/jfltexx
- it's one of the best supported phones.

Although I'd have a spare phone on hand in case something goes wrong so you
can still make calls.

Theo