Proto's right. It would only be small-universe syndrome if it were a coincidence that all these run-ins happened. But none of it was coincidence. They weren't random cameos like, say, revealing that Quark was a crewman on the Ferengi ship that destroyed the Stargazer. I focused specifically on characters that were established in TNG as old friends or acquaintances of Picard or people that he handpicked for his crew, and I tried to show the foundations of those canonically established relationships.
And really, considering that the book spans nine years of Picard's life, it would've been implausible if he hadn't encountered multiple familiar characters over that span. The very nature of the book required it. What would've made it small-universe syndrome would be if Picard had met all those characters at once. Instead I made a point of spacing it out.
I'll admit that including Janeway was a bit of a reach, and my editor resisted it, but there was indication in First Contact and the novel Homecoming that Janeway and Picard were previously acquainted and on friendly terms (and Janeway's Picard imitation in "Scorpion" offers canonical support that she knew him personally). So it falls under the same category of showing the beginnings of a relationship established onscreen. Sure, I could've easily used an original character there, but it was a major role in that phase of the story, calling for a character the audience would be invested in, and Janeway's pretty much the only prominent character who was active as a Starfleet science officer at that point.