I think Daniel T Gaming did a Big NSMBU Mario costume on his Youtube, so you can ask him if you can use. Though, your current sprite is amazing, so I don't see why you would change it.
Old 3DS XL info:
I used to have a 3DSafe A9LH V11.3.0-36U with Luma in my CTRNAND. Since ReiNAND Reibooted and Re-Reileased, I "switch"ed back.
Right now, I have boot9strap with Luma 8.0 as my CFW. Though, I'd like to see other CFW's for B9S.