I got to use Paint tool SAI until around 2014 when I came upon PaintStorm Studio, it works and feels similar but it is incredibly much better than SAI. It also features some of the tools found in photoshop and sketchbook pro as well (the rulers are and the rope (lazy mouse) are the best imho) so it is a really great program and It is also very cheap, I highly suggest you to try it out.
Another program you can use is Clip Studio Paint, really great overall program that has a huge deal of tools similar to photoshop as well as really cool brush customization features. It is worth much more than PaintStorm though.
I wouldn't go with an ipad and procreate just because everyone else and their mothers are using it, most of it is just hype.
About your original topic, yep, it is a cmyk problem, you can avoid color conversion issues if you work with less saturated colors, so try avoiding picking colors at the top right corner, this should reduce a lot of the problems when converting to cmyk.
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