OnePlus 6
I know it’s from 2018 but it’s still the most modern smartphone with an almost perfect linux mainline support, hence the very good support by alternative mobile operating systems (not only LineageOS, but also PostMarketOS or even mobile NixOS!).
It still is powerful enough for all usages, it has a good enough camera, great battery and the design still looks quite modern.
That’s not very telling without more data. Most x86 laptops consume 10W idle, x86 desktop PCs 50W idle. Double or triple that for moderate usage.
RPis? 5W max.
It’s not only about electricity costs, it’s also about heat dissipated in your room, fan noise and more generally waste.
Changing your phone just because it doesn’t have security updates anymore is a bad idea. Not only those security issues are very likely to be almost impossible to encounter in real life, it will also create more e-waste we definitely don’t need.
As suggested, install a custom ROM if you’re worried, otherwise just enjoy your phone it will still work perfectly for 2+ years.
Latest version of Jerboa with a smaller instance is very smooth
navidrome + ultrasonic