There have been some great answers, specifically: concentrate on making a fun game instead of working forever on your own developer tools (like an engine) and focusing on marketing.

However in regards to, "when making the transition from hobbyist to full-time indie?", here's what I think is the biggest mistake:

-- Do not go fulltime indie unless you have enough money to keep you afloat for years. --

You should hope for the best (selling tons of games) and prepare for the worst (losing money every single month for a long time). You don't need the additional stress of not knowing if you'll be able to afford rent when trying to make it on your own. Just find the money somehow; be responsible with your dreams.