Start with FUTO Keyboard if you want swipe, corrections and voice on the phone. HeliBoard if you want every key tunable. Thumb-Key if you type with one thumb.
Will I lose my dictionary or accounts?
No. A keyboard does not hold your accounts. Words you taught Gboard stay in Gboard. The new keyboard starts with its own dictionary.
Can I go back to Gboard?
Yes. Tap a text field, open the keyboard picker, pick Gboard. Or turn the new one off in Settings, System, Languages and input.
Does FUTO Keyboard need the internet?
No. Swipe, corrections and voice run on the phone. You do not make an account.
Is there something for one thumb?
Thumb-Key is a compact grid you type with one thumb, with swipe-to-letter keys.