Well, a pop-up warning is definitely better than simply a like somewhere in ToS. I don't mean the possibility of someone winning the whole bankroll, of course! I was talking about limiting the maximum bet in the way so that you can afford to make the full payout to the player.
And as for a bigger bankroll - have you considered accepting investments? Like, when people can give you their money that'll go into the bankroll, and they'll be receiving a certain percentage of money from the bets people lost to your casino. As you're new, I guess some might find it suspicious and risky, but maybe, later on, it could attract people and be beneficial both for you and for investors.