Unfortunately my current employer is fairly unopen to the idea of any kind of commercial development on the side.
You can create your software on the side, offer it for free and only quit your job and start charging when the software takes off.
The hardest and least predictable thing is finding product/market fit. I don't know how to do it (other than launching a product and waiting to see if it takes off) and while I lurk in startup-related forums like r/startups, most of the ideas I see seem very unrealistic (from the point of view making money).
Second hardest thing is finding users (i.e. marketing and such) even if you have a decent product.
Also, beware procrastination. Many people underestimate how easy it is to not work when you don't have to. You might be an exception (you can test it by doing 8hr of work on your project on Sat/Sun and seeing if you can keep it up for e.g. a month).
Otherwise you are opening yourself to lawsuits from employer.