Active servers sound good on paper. But keep in mind it depends on the kind of player you actually are.
A sea of people can drown you out, so if you have issues with anxiety then you would need a guild and finding a good guild can be difficult.
Larger servers have more people playing the game, but a lot of them are actually just lobbyratting. Still, in this area if all you do is queue up things and never socialize this might technically be better for you for speed.
But the bigger servers have a darker side, doxing, stalking, and a lot of anti LGBT+ and racist folks run rampant. Because the larger the server, and one without moderation like PSO2 is, it begins to cultivate into a hostile community in the more popular blocks. Be very careful of popular blocks on the more popular ships. Use proper online security tools and as word of advice, never use the same name in-game that you use on social media because people do like to stalk.
As for smaller servers, sure you might find harder queues but it's easier to socialize in because a crowd of personalities isn't drowning you out. Usually all (not literally but most) the toxic people like to flock to the popular blocks cause it gets them the most attention. So there's a nicer feel in small communities that I've seen in my times on MMORPGs since I've been playing them in the early 2000s and I have about 20ish MMOs under my belt.