I mean no offense by this, but I'm trying to fully wrap my head around the problem you have with ads. The only way anyone can tell that you're looking for something (either a person looking for a guild, or a guild looking for more people), you have to put your message out there. The only other way I've known guilds to form organically is by happening to group up with people, forming a friendship, and then deciding to create a guild together. But that takes time, and such guilds have pretty much formed their group dynamic and aren't really open to new people anyway. So unless you have the time and the patience to network like that, the only way you can get into a guild is to advertise yourself or seek out advertisements.

Is it the cookie-cutter city spam we often see? "Super fun guild looking for friendly members! We do WvW, PvP, PvE, guild missions, lotteries, and everything else you could ever want from a guild ever!"? Yeah, I agree, the city spammers usually aren't very selective and you can end up as just another influence node. Which is why skimming forum/subreddit ads can be useful, or, as you said, posting your own very specific ad. It's a bit more selective.

The only other option I can think of, which I've had some limited success with, is checking out /r/GuildMissions. A few of the guilds who post there usually do so because they need to fluff up their numbers to ensure success with their missions -- so you know they're not too big. And they'll often invite you to stay and try them out after to see if they're a good fit for you. It's still somewhat random, but at least it's an opportunity to try a modest-size guild that's active and adventures together.