I think that's the problem of making games: There's always so much more to put in and never enough time ;)
And I agree with you, you're not going to learn unless you do it. For a short jam, I would say it's easier to use existing assets for the majority of what I need to be able to focus my attention on the main assets, like characters, and get those right. By the way: What helped me getting into pixel art was restricted color palettes and tiny size. It's a lot easier to produce consistent results with just 4 colors and an 8x8 image. And once I had a hang of that, I was able to scale and build on what I had learned.