I will also echo most of what Zizaco says. I think some of us are trying to schedule a year or 6 months of a jam "circuit" to test ourselves and level up, but also refined ideas that could be put up on Steam or other ways for us as developers to grow too.
Although I agreed with the sponsor being the best approach, an alternative suggestion is to charge for the submission not joining.. and split the pot for the top 3 if it's large enough, and you take some % cut something like a fighting game tournament would do. Idea image You could create some interesting content around it but anyway... a monetary incentive could help develop the community as well as give some "skin in the game". Just keep in mind that you will also start to attract some bad actors.
Thanks for your content and for putting on this jam