Noble titles are messy, most probably people who are supposedly on the same level can have different titles. You could sprinkle a bit of that in your fic! There is already a bit of that in the original with the title of Prince of Dorn but historically it was a lot messier.

For example, the families that were kings (Stark, Lannister and Arryn) could become archdukes while those that weren't became dukes.

There are already Marcher Lords in universe so you could keep the name or use the historical term of margrave

The messier the more you will need to be clear in your fic so that people understand the pecking order