Since there’s no Arch Linux ARM port for the Librem 5 I am aware of, you would have to go with postmarketOS. Despite Edge seeming more fun, in order to avoid breakage on updates, I recommend you to go with postmarketOS stable - a new release of that will be out next month.
If you want to try it before switching, make sure your Librem 5’s u-boot is up to date and you can try postmarketOS from microSDXC (which negatively affects app start times, so it will seem slower than it is from eMMC.
Regarding KeePassXC: It’s just not mobile friendly, and they have no interest in making it adaptive, as documented in their issue tracker. I personally use ‘Secrets’, a GTK app - it’s only downside is that it is slower at unlocking/decrypting the database.
That said, did you also install ‘qtwayland5’? It and using Mobile Settings compositor tab may help with making KeePassXC more usable, if you want to stick to it.