If you want to know about the really small indie ones that are less for actual use and more for just "I want to make a game engine" I would check out the various gamedev Discord and Reddit channels. There's always people showing off their work.

Most of the people who follow this advice will never ship anything. Stop worrying and do what you want. It's okay to reinvent the wheel if you want to learn how to make a wheel, or if you just find it fun.

(But remember, if you want to ship fast, or at all, usually better stay with the common wheels and sizes)

Not sure I agree. I have always done both.

Ostensibly I am writing a game but not using someone's engine. And then my own "engine" comes out of the effort.

Then I write a second game and pull over the rendering-sound-etc-code from the first game. This "engine" gets further refined as a result of being pressed into service for the new game.

Often it improves with each iteration and I move the newer version back to older games and refactor them as required.

No, 343 Industries, you don't need your own custom engine. Just use Unreal.