August - I love this name! It’s so handsome and classic and the name of a favorite literary character of mine. I really don’t think it’s that common - it’s currently outside the top 100 (#167 in 2019). I also think it sounds absolutely wonderful with Perry as middle! It likely will only get more popular though, so I understand your hesitation.
Marlow - this one is nice, but I don’t love it. It’s not as classic as August but it’s very fun and more unique. The biggest problem I see here is that it’s becoming more popular for girls. I also think it sounds a bit too modern next to Perry or Douglas, though it would sound fine with either.
Cormac - I love this name! It’s got some obvious Irish connotations but I don’t think that’s a bad thing. Lots of names have Irish inspiration (McKinley, Finnegan, Niall, Brady, etc.) and I honestly love this trend. I think Cormac is strong, has great nickname options, and pairs wonderfully with either middle. I’d say it’s actually my favorite of all your choices, but it matters most whether you think it’s too Irish.
Merritt - again, this one is nice. I’ve really only seen it used on girls and I think I’d prefer it on a girl, but it’s a lovely name overall. I think I’d like it more if you didn’t have August or Cormac as options, but Merritt just doesn’t seem to shine as much as the others on your list. I also don’t think it flows well with either middle name.
Whitley - I like this one, but again it feels feminine. Whit is a handsome nickname though. I’d prefer Whitman as a full name, just because Whitley/Whitney have gained such strong feminine associations over the years. I do like it slightly more than Merritt, but again I think it pales in comparison to the others.
Rye - all I can think of is the bread. Sorry
Shay - I love this one! It’s a great name for either sex and I love it’s simplicity, straightforward pronunciation, and easy spelling. I still definitely prefer August or Cormac, but I think Shay Douglas is a strong combo.