Get a second hand business laptop (dell latitude, lenovo thinkpad, hp elitebook), replace battery where needed, install linux yourself.
If needed you can pop by the seller with a USB drive with Ubuntu or suchlike to quickly test compatibility.
Business laptop because of decent parts and guaranteed to be self serviceable.
As for power saving, I saw this pop by a while ago: https://medium.com/@amanusk/an-extensive-guide-to-optimizing-a-linux-laptop-for-battery-life-and-performance-27a7d853856c
I’ve upgraded without issue on my own instance, lemmy.zaggy.nl, it’s open for testing, break away, I have snapshots.
I apologize for the ongoing nonsense topics and Dutch.