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Regarding (1), that's on your keyboard, which you can choose. Maybe you can give Futo a try? https://keyboard.futo.org/

Why does it need its own F-droid repo?

Simple Keyboard is on F-Droid too. Supports moving cursor via space bar.

Same for HeliBoard

Because the code is not provided under a free/open-source license, and therefore does not meet the requirements for the main F-droid repo.

Open source with limitations on commercial use.

Technically it's "source-available", not "open-source".