when I was running my Oneplus 6T with a heavily degraded battery on pmOS, I got days of standby out of it, while Android couldn’t last 12 hours. in both cases I ran it without modem/cell connectivity, just wifi. degoogled android, no microG no nothing. still, no way it could do sunrise to sundown.
on pmOS, when I press the power button, the thing went into standby. didn’t do shit in the background nobody asked it to, didn’t sync to no clown, nothing. for me that was perfect. your use case might be different.


go to gsmarena and filter phones by chipset. select something like Snapdragon 845, a widely supported SoC that is at the cross-section of hella old i.e. cheap but also super capable; it’s doubtful you’ll ever need anything faster than that. then see which of the filtered models you can get for cheap. finally go over to lineageOS and see if it’s got current support.