Perhaps you should consider getting a fairphone if its available in your market. Having a user replaceable battery is much better than going back to the OEM.
Maybe your battery issues is due to li-ion chemistry used in Samsung/LG instead of li-polymer more commonly found in Chinese OEM.
https://www.androidauthority.com/lithium-ion-vs-lithium-polymer-whats-the-difference-27608/
No just 4 OS and 5 years for security updates.