Heya there and welcome to CAP.
It sounds like you are having good success with the freerolls. imho, I would stick with this idea. I wouldn’t go down the casino affiliate route with this site.
For one it will confuse your users and second of all the casino revenue wont be large as you are trying to get people that play for free to join. So you need to convert your freeroll members into paying members.
You can do this by making them perform and action before they get to participate in a freeroll, i.e. playing x ammount of raked hands. You could ask them to join a casino and deposit but I think this is problably a harder push.
Once your membership gets large enough you will be increasing your freeroll prize-pool and also your entry requirements. more $ for you
Good luck
Bart