I can't provide a roadmap, but something that helped me was to review the code for axe(4), the driver for my ethernet dongle.
I'm also interested in learning more about writing device drivers on OpenBSD. Some useful links I've reviewed before:
https://www.openbsd.org/papers/eurobsdcon2017-device-drivers.pdf
http://thetarpit.org/2022/notes-on-openbsd-device-drivers
