But also disclose that you got scared off of the $15k/fee and haven't tried that route.
In my experience the right recruiter can combine the utility of the "First-degree friends" and "Referrals" sections you list out.
I used to be anti-recruiter until we made it a point to "recruit a recruiter" and qualify them as hard as we would if we were looking for an internal role.
Interviewing recruiters through a process landed us a recruiter that is well known and respected in his niche, has built trust with dozens of candidates, and delivered strong hires.
We've gotten 6 amazing people in ~6 months and from an investment return basis I can't think of better money that we've spent.
The product side improvements that the new team members are delivering + reducing the brain damage you go through with bootstrapping hiring are high return investment categories in my view.
And the punchline is I think you $15k/hire is off - we've averaged $45k per hire (30% of first year) for engineering & sales talent in the $120 - 200k/yr range.
PS - As others have commented the "We spent less than $1,000 on hiring" should probably be amended, it weakens the rest of what is a very good article on boostrapping hiring!