Well, there are always ways to limit how much money one can make through gambling grinding alone: as I mentioned, I reduced the max size of the bet, which might already be enough, if not too much. There are other approaches too: you mentioned an idea I had with Sonya "nudging" you into stopping for the night, or limiting the bet size further but increasing it with each consecutive win in a session, so that you actually need some luck and can't just save-scum your way into riches too smoothly.
It's also worth mentioning that you shouldn't really need to grind to begin with, and if that's the case I can always tweak quest rewards upward, or add more money/treasure to find around the world, instead. Do let me know if some quests felt too stingy or if there is something that is priced too high.