Nick,
This is a long issue.
Is it really so terrible with brtfs or is it just a question of configuration?
Have you heard about: fatrace - report system wide file access events. Detect what does wake up your harddisk - search for fatrace - I can only post 2 links.
Do you know armbian? There was a guy: TK who was really into performance and perfection. He took the time to go into details and find troubles some parts.
Now this TK really liked btrfs. For example: Some storage benchmarks on SBCs - Reviews, Tutorials, Hardware hacks - armbian forum
The tipps I mention above are from armbian.
About the wear out of the SDcard, it is a long post, but if you start reading here it is not much, but quite into details: And besides that I really see no benefit : Learning from DietPi! - Off-topic - armbian forum
I wonder if you can find something to measure or improve the situation?