D.L.
Picking a name is so hard! When we were picking names, I said the ones we were coming up with - thinking of different situations, wondering how it would sound if they were in trouble, when the teacher is calling the name, when they announce the name at graduation, etc.
Worried about Bentley Morgan - they're both cars, though I do think Morgan is cute. I'd consider Morgan a boy or girl name, though I've heard it more for girls than boys. Is Nicholai a family name? (We have a Nicholas) Here are some boys names that I thought would be good since you have an Ian already -
Macaulay, Bryce, Colin, Evan, Liam, Logan, Sean, Shane, Ryan and I think they all go with Michael or Morgan as a middle.
I love McKenzie Rose but personally prefer the spelling of Mackenzie - I would worry about about how popular it is, though (but also wouldn't let it stop me if that's the name you want - it is a beautiful name!) Are you comfortable about how Gracyn will be pronounced? I assume you're pronouncing "Grace cyn" which is very pretty. Agree with the middle name Mai vs Mae. (My or Mae) Love that Mae is a family name! Also liked the Hannah suggestion especially since you have a Hailey (very cute, BTW!)
Also thought the family tree idea was good. You might find something you really like there! Another fun thing might be www.howmanyofme.com.
Good Luck! (Hope you let us know what you choose!!!)