We use Flask and Python with Postgres, and for the payments we use Stripe. Basically everything is automated, apart from withdrawing your winnings. That part is still manual for now as we haven't had the time.
At the moment we're definitely looking at IndieHackers and ProductHunt, but also using Pioneer's tournament aspect.
We have some basic weekly challenges right now that help boost your score to incentivise community feedback (these are only be visible to users). Adding competitions is not something I'd thought about, but it's a fantastic idea.