Yeah I was having exactly the same thoughts. So first of all: forget screen brightness apps. Even the one requiring root are just…yikes. Functional workarounds, but unpleasant. They basically just keep a permanent new slider open that forces the brightness.
So I collected a bunch of resources yesterday to look into altering the config files myself. Like adding a few more values, or just modifying the lowest three.
What I found was this post: Official LineageOS 18.1 for Fairphone 3/Fairphone 3+ - #198 by TeamB58
It’s about altering a config file for brightness in LineageOS, but there should be an equivalent for Fairphone OS. But so far I haven’t had the time to find the file in Fairphone OS, or yet alone modify it.
So feel free to look into it.