Hundret percent with you there. Don’t know how much of a hassle it would be to make these changes to the Fairphone OS, but it would be pretty cool to have it.
However if this is something of importance for you, I don’t know if you’re aware but there are existing apps out there that can do this for you. On my previous phone I used Go Launcher, I was able to add/delete screens and easily change their order. There are a lot of themes available for it, also to change the lock screen. Don’t know if you can get it to show notifications on the lock screen though. I don’t have any experience with other launchers, and I’m not using one on the FP now.