Yes, you can have a side job, but you will have close to no social life. The best part about a side job like coding is that you can do it on overnights in your hotel room. However, you will miss out on going out with your crew for dinner and beers, which to me, is half the fun of being an airline pilot. To each their own.
How to live on $20,000? There are a lot of answers for that.
As to which regional to go to, I'm sure it will change by the time you are ready to make that move. For most people it comes down to pay, domicile, upgrade time, and short term stability of the company. Secondary would be flow-through programs, sign-on bonuses, and certain contractual rules. Again, I am speaking in generalities.
Lastly, you make it sound as if upgrading to a legacy carrier is a foregone conclusion if you work for a regional. Let me be the first to tell you it is absolutely not. There is zero guarantee you will ever work for a legacy carrier. Count yourself lucky if you do.