My pleasure.

Re: commercial AuthN/AuthZ services - I definitely understand your rationale (though I've seen some passionate opinions of the opposite nature; not only Auth-related, but generally, in the vein of "outsource as much as possible of your infra to external services" (BTW, I strongly disagree with this stance for most cases). Returning to Auth, I guess, the right answer is "it depends".

Re: multi-tenancy - Yes, there are certainly various approaches, but most, if not all, represent those three that you mentioned. I thought that it would be nice to have your course expanded with demonstration of how relevant multi-tenancy design decisions (ideally, with all three options) affect other parts of the codebase and integrate with them. Perhaps, I want a perfect course, but since the focus here is on SaaS, multi-tenancy coverage IMO makes perfect sense.