Ways to become an illustrator:
1, you can go to a sketch class to learn sketch after work. Tell the teacher about your situation. When I just graduated, I just joined the work and learned the colors after sketching. I studied gouache at that time.
I suggest that you don't have to find a particularly expensive class, just find a teacher with enough time and patience. Sketch is very important! Remember this. Practice if you have nothing to do.
If you feel that you have enough time, you can learn some software later or at the same time.
4, PS AI will. Learn to be a painter first, and then learn PS AI and PS AI. You can specialize in illustration and painting.
5. The mouse painting you mentioned. Personally, I strongly disagree. Without a tablet, it is difficult for an illustrator to express something. Even if you love AI, it is actually faster and easier to operate based on numbers. I mainly draw AI now, and the graphic tablet is really much better than the mouse. It's not that the mouse can't draw well, but that the graphic tablet is the basis of an illustrator. You must study.