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Bustadice is good but as far as I know, there's not flash bets on this platform though they have auto bet but not a flash one that OP is looking for. If I were to OP, I'll stick to Wolfbet since that's also a good site and its hard now to trust another gambling site especially if you are new to it. I've tried flash bets before on Wolfbet and it's really working fine.
I haven't played in Wolfbet yet so I don't know much about it but they have a signature campaign here if I recall so that can only mean that they're somewhat reputable.