STEM is a very broad thing - science, engineering, math - so what is your specific interest anyway?
Truthfully if you want a job in STEM, you NEED to go back and get a STEM degree. There are too many people already out there with degrees in the field that you have no hope unless you get some big favors by some very important people.
However, if you want to dabble in STEM type stuff, there is no reason you can't get involved as a hobbyist. Many self-taught people code outside of work on open source projects. There's the whole Maker movement with plenty of resources for the amateur. There's amateur astronomy, rocket clubs, cube sats, and so on. In fact, if you become a guru in any of these hobbies, it may open doors to a technical career.