> How many were successful in contrast that just used an off the shelf engine?

Yeah there's successful games that used an off the shelf engine but saying very few roll there own is plainly wrong when a bunch have. And that's without mentioning the whole flash era...

Around 18k indie games are released on steam per year.

Around 300 of those are successful, making over $1 million [1]. VERY few of those successes are using custom engines. Your list is probably close to complete, being far less than 1%.

[1] https://www.shanethegamer.com/research/indie-games-statistic...