In case you haven't already, you might want to check out the McKusick courses (available as videos). They're not cheap though, maybe you can find pirated copies.

I was fortunate enough to see some back in the VHS days when someone in a local LUG convinced their employer to buy them as a way to support FreeBSD and give the team something to watch together (and recruit from the LUGs via hosted viewings no doubt).

https://www.mckusick.com/courses/