OP, if you are going to do professional work and focusing on illustration, some of the better alternatives is CLIP Studio Paint. It has the easiest transition and most robust features comparatively.
But if you are into image editing, you are out of luck. PS is the best image editing software currently.
I know 120/yr seems pricey, but i look at it this way. PS used to costs 600 as standalone, and people still use it anyway because it's the best. So at $10 per month, I just take it as if I'm using PS for 5 years before I buy a new version.
I know it's a lame analogy, but the reason others price their software so cheap because they know they can't compete on features. I'm all for a cheaper software, but until others can provide similar feature, PS gonna stay that way.
ps: I've used CSP, Painter, Photoshop.