Thank you to everyone who replied.
It looks like including evidence (flight receipts, boarding passes, hotel receipts, photos) for the most recent visit, while also listing the other visits is the best method. I will also include photos of other visits as evidence of an on-going relationship
We wanted to wait a little bit to get married, especially since we're still students. Both of us will have graduated by the time she arrives (hopefully). I also have a co-sponsor.