Samsung Keyboard literally lets you design your own keyboard layout in a surprisingly robust and rich way. I don’t know if it’s available on non-Samsung phones though, and I can’t wholeheartedly recommend it because it has a bunch of flaws and quirks. For example, every once in a while it seems to do select all + copy + paste, without you going anything besides typing normally. This can scroll the text to an inconvenient place, and remove special formatting. On YouTube if you’re replying to a comment it destroys the username you’re replying to, replacing the special highlight with just their name in plain text.