There are a few big bottoming debut scenes, like Paddy O'Brian, Jimmy Johnson, Brandon, Nixon, Mark Long.
This blog has been tracking bottoming debut for years. One of my favorite blogs that I check daily.
Bottoming Debut | MenSparkle
Men.com used to have a great "top to bottom" series when that lesbian couple were directors. They are talented and they know the taste of gay porn fans. Too bad they left early. After Diego's BF took over that position, Men.com started to focus on parody/drama/comedy series, even with females. It's a big fail.