I'm a huge fan of the mini games in RDR such as dominos and poker but I have to say for realism that time period no one played texas hold'em. The most popular poker game in that time period was 5 card draw and 7 card draw was the choice at casinos. Also there were casino's all over the place, they'd play all kinds of card games, roulette and even 1 arm bandits!
For realism sake someone needs to create a casino mod and make it high stakes 😁
Before you say "do it yourself" I'm too lazy to try!