40 hours per week is more than half of most people's waking hours. That's a terrifying amount of my life to have to spend on something I don't _absolutely love_ doing, and I sincerely hope I can find a way out of it sooner rather than later.
So my point was just to not bet full-on on two horses at one. If your priority really is your side-gig or starting your own business, then do not run too fast in the hamster wheel also. Put yourself in a position where it might not be the most fulfilling work, but it serves the purpose of giving you economic room for pursuing your own business.
Interestingly, for myself the more intense the day at work is, the more energized I feel when I get home to work on my own passion projects and businesses. Since my role has been relatively autonomous, I'm the one setting the intensity level and if it's a high pressure day it means I bit off more than I bargained for.
To avoid the after-hours productivity slump, I try not to let work get "slow" by taking on new projects, asking other teams what problems they're encountering, and finding new tools to add to my arsenal. That being said, there are still days that seem to crawl by and very little is left to be done. Those are the days that drain me mentally and physically. Some of the people I've worked with have talked about how they spend their workdays pretending to work while actually playing mobile games or whatever and how it makes the day go by faster. I'm genuinely fascinated by what I perceive to be their complacency and lack of ambition.
As background, this is my first role at an organization I'm not a founder of and I started there three years ago at the age of 27. Starting at age 14 I had my website network which was racking up $500+ a month in server bills, so the entrepreneurial lifestyle (rollercoaster ride) is my definition of normal. As a first-time employee, I've had some embarrassing moments while learning the rules and etiquette which is probably common sense to everyone else.
You will probably have to decide what is most important to you. You cannot chase and strive for promotions, extra responsibility etc. on your day job while still wanting to do your own thing in your spare time. Perhaps you need to look for a job that has less pressure and perhaps less pay, but which does not drain you.
I have just had to go through the same realization myself that I cannot chase on both fronts. And my priority is my own projects, so I will have to course correct.