Fusion 360 is free for hobby/startup use. I’ve used it for all of the designs I’ve made, including one that is for sale here at YYE.
It took me a couple months of occasional practice doing throwaway designs before I found a winner, but with 3D printing you can actually test your designs much more quickly, and potentially, learn faster.
I wrote a whole thread about my learning process here: Yoyo CAD & Prototyping Adventures
Are you already pretty familiar with 3D printing? There’s some tricky parts of it that aren’t really apparent at first, especially with yoyos. You’ll probably want to use 100% infill, and it should be concentric circles instead of a waffle pattern. There should also be a setting or option to reduce z-level seams that is important to use. The trickiest part is making the guts - it’s easier for fixed-axle types, especially since the overall weight/width of the yoyo can be lighter, which is suitable for that style of play.