how can I allow 'simple' user input of subscript / superscript in rich text box?

like subscript & superscript buttons

I'm afraid the Rich textbox doesn't have an option for your users to enter their text as superscript nor subscript.

However, the TinyMCE widget might do the job https://widgets.jotform.com/widget/tinymce

This widget will allow your users to format superscripts and subscripts, like so:

Here's a guide on How-to-Add-a-Widget-to-your-Form 

I understand you want something "simple" but after checking different widgets, this is the only one that offers the easiest way to implement super/subscript.

Hope that helped somehow :)

Looks perfect! Thanks for the quick reply, I'm excited to give it a try.