I am not saying you are doing this, but just saying it is not provably fair.
Wow, that's true. I've seen other websites use this provably fair algorithm, so I did the same, but you are right.
The solution you propose is good. At the end of a jackpot, we could check what is the current block, and then wait until 2 blocks next, get its blockhash and mix it with the server seed.
Because of this and other responses (which had constructive criticism I appreciated), I've paused jackpots on the site for now.
If you looked at the website, you saw a lot of bets running over and over, making it seem like there were a lot of users. In reality these were bots I had to 'fish' people into betting, making them think they were betting against someone real. This looks bad, but I assure you they were fair: if a real player won the game, he would win money from all the bots that bet in the jackpot. There was also no code for the bots to 'steal' the player by betting more - they always bet random amounts.
I did this because this model of jackpot site only works if there are enough people currently betting, otherwise you get your bet blocked until the jackpot ends, and it's boring.
I did have some people sign up (all prior to this post) and depositing. They won and lost in the jackpots, some withdrew more money than they deposited, but at the end I didn't make any serious profit (there were probably 10 real bettors ever).
So, I'm rethinking things. This project did take a while to get finished, but I also have college going which will take some time from me... IDK, maybe I leave this be for now and return later with another idea...