are there any non-systemd DBus options anymore? I had a hard time a year ago setting up Gentoo because there were no maintained options at that time.

There are. Gentoo has actually always used the FreeDesktop reference implementation instead of sd-bus (the systemd implementation).

FreeDesktop also maintains a list of implementations (also including bindings so you have to read to find which ones are full impls vs libdbus bindings).

https://www.freedesktop.org/wiki/Software/DBusBindings/