In case you're still thinking of that Sony, I've found mine are quick to update, still got an X Compact that is on the October security patch and 7.1.1. they might fall in the price range you're after.
Also, have you considered Moto? Seem good range of prices available, and they (used to) update fairly frequently. Others may have a better idea of current pace.
Is there any 5X still around, a nice refresh of the phone she already has.