Adobe rejected these illustrations due to quality issues

Technically this is not an illustration.  It's a photo of a painting.  🙂

In my experience, hot press untextured paper gives better scan results which leaves me out because I prefer to paint on cold press cotton. 

If you use a flatbed scanner, clean the scanner glass thoroughly.  Lay a dark cloth and some books over the painting before you scan.  Use your scanner's highest quality settings.

There are several successful watercolor artists on Adobe Stock.  Examine & compare your work with theirs to see what gets accepted.

https://stock.adobe.com/search/images?k=watercolors 

I don't think AI can help you much.  My guess is it would do more harm than good.  But you could try it on an image copy and see what happens.

Good luck with your next submission.