Mac and cheese separating

I'm struggling a bit with my Mac and cheese separating a bit after going in the oven. I have a couple things I suspect are the problem so l'd love help and input. Before anyone asks, yes I always grate my own cheese fresh lol. The sauce before it goes into the oven is perfect consistency and texture.

Here are my theories as to what's going wrong:

I wonder if my butter to flour to cream ratio could be off? 5tbs butter, 2.5tbs flour, 2cups heavy cream, 2 cups half n half

Potentially the roux is too hot when adding the cheese

Typically I make ahead of time so once it's cooled it goes in the fridge and then it reheats in the oven and that's what causing the issue? The times that l've popped it in the oven right after making the roux have not had this issue.

Please help!