Hi and welcome!
You can disable the on-screen keyboard in the settings page → Settings → Settings → Keyboard behavior → Off. You can access the settings page by clicking the glass icon on the top right corner. The same page also has the font size slider. I’m not sure about the range of it (never checked how it calculates the limits) but if it needs more range, please let me know!
There are other settings too, feel free to test them out! I personally always disable the transparent background to increase readability, but we are wondering off to the personal preference territory 
Another tip: if you find BusyBox too restricting - and you are feeling adventurous - you can install actual Bash instead, it comes in the Jolla repos, but it requires some pkcon fu. Please see e.g. this thread for discussion and commands.
There’s also Fish Shell, packaged by yours truly, but if you do install it, no not make it a default shell! You have now been informed 