WatchOS 9.whatever the latest. I downgraded from a S6 about 4 months ago because the S6 was having issues and all my local store could offer was a SE2 and $75 so i took it.
You are not on the last eat firmware and are complaining about battery life. At least update before you bitch about shit. I assure you as a se 2 owner the leastest version is quite stable. If you just wanna bitch without trying the most basic things then go ahead