Gentoo. Not an Arch fork, and uses OpenRC by default. I use it and love it. Portage is the best package manager out there, imo.
You can still get binaries of the really annoying things to compile, like Firefox. Otherwise, it's all source-based.
I'd advise installing it in a VM or on a spare computer first to get your hands around what it is.
In your case, you'll want to specify the following flags in you makefile:
OpenRC, -systemd
You'll add a bunch of others in there too depending on architecture and personal priorities.
Follow the handbook. https://www.gentoo.org/get-started/
There's also Calculate Linux, which is basically Gentoo with a graphical front end, but I think it's Intel only. CLI is more fun anyway.