If you enjoy Charlie’s, you will definitely enjoy Kahneman’s Thinking Fast and Slow, especially the part about being an « expert »: a few talks in empty classrooms in a famous Uni, a radio show nobody knows and voila, you get some cred!
The big lesson for me is know what you are getting into. Look at the OP - he spent every spare hour he had. This is no joke. I have done something similar in the past for a time and I ended up constantly running into conflicts of priorities between that and personal life. I ended up wasting a few years, in both personal life and professional life, although the former hurts much more. This is how I ended up in a scenario where I have nothing to show and nothing to lose. I just hope I can do it all at some 50 hours per week total, where the product is just a part of the day job (promotes the consulting offering) and lower the volume of paid work as I need, if I want to have more time to make a big move with the product.
The golfer won't regret their day on the course. And if you fail on the passion project it won't feel like a fail.
I have another idea too. It's the win anyway system. Pick something that if you fail you use those skills at work and get ahead. E.g. the side project is also the training for the gap in your career.