Time to do what I promised:
You may want to look into Sxmo and their wvkbd, it seems to very close to what you seem to want in terms of layout. If you are considering to make this keyboard work with Phosh, … I have not come across a success story about that. Here’s a pointer on how to start: How to use a different keyboard?
Regarding the Firefox issues: Purism’s fork of mobile-config-firefox has its issues, I find the upstream variant to work better. Remember to adjust the firefox dir in the makefile before trying it out, IIRC it’s firefox-esr instead of firefox on PureOS. If you have patches, I am sure the project is happy to discuss them (please don’t make people copy stuff out of forums like in another thread 
Regarding the Camera: When I got my first Librem 5 in early 2021, it did not work at all. At the beginning of this year, it still was pretty much: You have to move these sliders manually, good luck with that. I am sure more progress is possible, and contributions are welcome. This is the project for the Librem 5 camera: Librem5 / millipixels · GitLab
Regarding scaling and apps: There’s this project I am involved with called LinuxPhoneApps.org. I need to spend more time on it, but I hope it’s still somewhat helpful to save time on figuring out which apps work.