Default behavior for Rehearsal Marks

Is it possible to have a default behavior for whether or not rehearsal marks use the numbers, letters, etc… sequence type?

Yes, in Engraving Options.
https://steinberg.help/dorico/v1/en/dorico/topics/notation_reference/notation_reference_rehearsal_marks_project_wide_engraving_options_c.html#notation_reference_rehearsal_marks_project_wide_engraving_options_c

Of course once you’ve set it there you can Save as Default and it’ll carry over to future projects.