I have the same problem. I think 2.5K is the maximum at level 2 (my level). I noticed this a few days ago when I saw 2.5K/3K fans attending my games. Funnily enough I still got the max +5% possession bonus. I have tickets at $5 a game and I still didn't get a full stadium when I changed it to $4

Also got maximum stadium features at my level. L4 parking and still don't get full stadium.

So maybe there is a cap at different levels