I’m not in a good position to give hard advice.
I rolled on Mirage Raceway and stuck to it - seems perfectly fine realm to me.
Some people love busy realms, others don’t. I only PvE.
Check this out for population expectations:
https://ironforge.pro/servers/?locale=EU
This is a hint about oversized / weak realms, too.
I did Google it, found a thread and literally every post was vouching for a different server. Maybe pick one, then search here for posts about it.
I guess you’re aware of what population usually means in terms of PvP, namely the underdog has instant battlegrounds but trouble questing.
Also apparently the biggest PvP realms are balanced, while PvE realms are quite skewed. I played on such a server as horde in 2009-2010, it was still viable with several serious raiding guilds, nearly instant BG queues. Wasn’t dungeoneering too much. I faintly recall Wintergrasp and Tol Barad being mostly alliance territory, though.
Two problems - 1) it says active raiders and 2) it’s about a month old.
Might be worth opening a similar thread.