You could check out SurvivalMC it might suit your needs.
It ticks all those boxes (except maybe the "modded" part). There's huge base's built in Survival. My base is a custom island going up to y=200 with a castle reaching height limit. And there's others with spaceships etc.
We also do events every so often which normally bring 20-30 players online at once (next event later today). - THE WALLS