Are you using Mac Finale v27?

I suppose that you - by your words “lettering (rehearsal marks)” - are talking about expressions from the category Rehearsal Marks - right?

Select the handle of the rehearsal mark, and go to

Expression menu > Edit Rehearsal Mark Sequence…

If you need to do this a lot, consider adding a keyboard shortcut, or consider using the context menu.

It is not clear to me what you mean by your word “numbering”.

- page numbers? - score or linked parts?

- measure numbers?

 I'd like to restart numbering AND lettering (rehearsal marks) for each movement of a larger work

You can duplicate the expressions rehearsal marks category as you wish. Each time it will start back to "A" (or whatever). You can have as many measure number ranges as you want, also. Just define a new section for each one and what number you want it to start on.

Personally I would recommend you make each movement a new Finale file