I did similar experiments before those two, but prepared the results less thoroughly. 2 in total with one year in between.
The short version of the result was: 0 conversions, but 40-50 more followers with 1000 ad zaps at 21 and 50 Sats each. You'd need to invest more zaps to get a somehow more representative result.
But that's just another form of cheap marketing, incentivized and morally legitimized through zaps. Meaning, people only accept it and don't classify it as spam because they get a few Sats for it.
The marketing approach that really works here is authenticity, I would guess. Spit out notes about your expertise for 3 years straight and eventually enough people will know that you're the XYZ guy. And for an extra boost, get some testimonials on Nostr (signed and authentic). And remind people by reposting them regularly. Though that would become borderline again; I would quickly unfollow such annoying people. 🤷