I just recently released a tiny game as well (http://declanhopkins.com/asteroid-quest/), but it was only put on my website and I made it in a month. Regardless, it felt really good to get a game out after years of trying. I'm working on a new version of it now.
One question - how hard is it to actually deal with the administrative, non-code tasks of turning your side project into your main business? I'm talking about incorporation, supporting clients, basically all the stuff that goes on outside the source code repository.
Supporting my customers ought to be the biggest time sink, but actually, I never needed to patch the game.
Organising discounts and Steam sales and things like that takes thought, but I actually really love that side of my business.
For my current project, marketing is going to be a big deal for me, but I learned a lot doing that for The Cat Machine, and for me it's as important as anything that goes on inside my Git repository.
Using cats was a good idea, definitely.
The music is done by Peter Nickalls (http://peternickalls.com), a friend of mine that I've known for years and years. Too talented a composer to be working on my little projects, but I'm thankful for his work. The music he's worked on for my current, somewhat more ambitious project is absolutely incredible though.
The key difference is that my project was a video game (this one: https://www.youtube.com/watch?v=YwXl8lDrxn8) and that takes a lot of temptation to do what the OP is writing about, rewriting stuff in Haskell for example. The interesting bit when it comes to side projects (and now my full time projects) for me isn't actually the tools or the methodology or anything like that, but the end result. I spent my teenage years building games that I never finished, and it made me quite miserable, because I so just wanted to finish something. Anything.
I remember the first game I ever finished was a clone of Snake. There was nothing remotely interesting in the implementation or in the design, but I was so flipping pleased with myself, I had finished something! And that let me slowly build my projects up to more interesting endeavours, and eventually to one that could be sold. Starting small was the key for me, as I learned two things: 1 - what was 'too much' for me to try and tackle at the time and 2 - self-discipline, not chucking the project the moment I got bored, or had a 'better idea'.