Also, I feel like developers will be much more willing to work on fulfilling these bounties if I can provide some way of ensuring that they will be rewarded the bounty upon completing the task, but I'm not really sure the best way of doing that. So if you have any good ideas relating to that let me know.
The best way of doing this is dropping this "bounty" system and just hiring developers?
Is it really that mysterious?
Never mind that it seems grossly inefficient and a waste of resources to have "competition jobs" where people are developing in parallel, racing for the bounty prize.
Also, you might want to discuss this with developers to find out the timeframe and compensation for developing what you're talking about.
I get it that you have a budget, we all do, but realize that your budget dictates the quality, speed and interest you'll find in developing these products. In my estimate what you're looking for versus how much you're willing to pay for it is wildly off base. For reference, I've been a part of small HTML websites that were more expensive than what you're offering.
Someone stated "Devs are hard to find" in the thread. This is absurd. Devs that are willing to work under customer dictated terms that attempt to bypass the marketplace norms, maybe. I don't know too many development houses that would think these terms are fair.
Feel free to seek it out, of course, but all of this nonsense regarding how to find devs and how to make them feel like they'll get the bounty are easily solvable via contracts and directly hiring firms that develop software. The institution I work for never has a shortage of vendors who are willing to accept our money in return for software.