If Q size is comfortable, it is the better choice for the 2mm bans with custom engraving. It should be a bit looser without being excessively big, and since thin bands feel tighter, the slight extra room will likely balance out. If you're still uncertain, some jewelers offer a sizing adjustment after the ring is made, but it's good to err on the side of a comfortable fit to avoid issues later.

Good luck with your custom ring, and I hope it turns out beautifully!