It seems like you're always winning if you don't double. If you want the player to feel exited to win you need to lower the chanses to win. If you plan to pitch it to a casino company, or sell IAP for more coins you need to calculate an RTP of around 94-95%. RTP = Return to player. Which means, in the long run, lets say over one million spins, the payout to players in only 94% of the money spent. Otherwize you will go bankrupt...
I work in iGaming (online casino bussiness) at the moment, so if you have any other questions let me know.
Otherwize the game is good, but you need to build suspension, by having some no wins, some small wins (lower than the bet) and a very few big wins.