I've never used Discord JDA, but I have used Discord.js which is a discord api for nodejs. I find that it works very well and is intuitive because nodejs is already well made for asynchronous tasks (things that happen at arbitrary times and in parallel as opposed to one after the other in a "stack" if you will). Nodejs is becoming one of the most popular web frameworks so if you want to learn anything web back end, I'd start learning node, not Java. As to what to make, just find a problem or a need, in this case, a bot you want for a server (maybe even ask some mods that you know if they have a want or a need for a specialized bot) and try to make it. Good luck, programming is a deep rabbit whole!