Vivaldi is a good option as far as privacy is concerned. Battery life related to a browser is a topic that does not come up. It would be interesting to see your rankings for your use case and system.
Not sure if this would help, or where your at in your busy life. You may have some experience with other operating systems. It sounds as if you can handle an affordable side project.
Have you messed with any Live operating systems. Tails OS has a good amount of documentation that would help you learn about building up a thumb drive operating system. You can build other Live operating systems as well. I just recommend Tails to learn a little.
We have members here who share computers with family members. They run Live OS. This could be an affordable way to learn other operating systems that meet your performance needs. Its an affordable option to use a “different” machine on the same laptop.
My use case for Tails is related to using computers which I have permission to use, but I do not want to leave a trace of my activities. Tails allows me to use these computers, connecting to the internet with my phone.
Some other systems I have on a USB, Kali, MX Linux, Fedora, instead of sharing a link to these OS individually check out https://distrowatch.com/