Can't speak to Phoenix specifically, but from someone on a similarly sized server (4500 vs 4900):
You will rarely if ever see RMT. The only real presence on Carbuncle is 1 guy who bots CP and a dozen or so mules in starter zones spamming sparks.
There are no mercs, and the only botting you will see is a handful of people at the common CP/EP camps botting by themselves or with their friends.
Economy is weak, but steady. If you really want any Su5, JSE neck, Su3 armor, Abjuration items, etc. you can find them, but if you're trying to sell things they won't sell lightning fast, and you'll have to sit on things for days, weeks, or even potentially months at a time.
Hope this helps. I prefer my smaller server, it's really nice to not have to have an addon to block spam, be able to farm abyssea whenever I want, have no queues for any content, and not deal with the merc mentality. If you aren't good at the social game and want to get endgame content done, it might be rough.