I would like to try that approach but I just don't have the energy after working all day to come home and start investing time chasing wildly creative projects.
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.
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.
But the TL;DR for the chapter on motivation and energy comes down to: work on things that improve your own life, set clear objectives, and work in small chunks. The book itself is being written in 30 minute chunks throughout my week because that’s all the time I can spare.
Of course, that's not really sustainable. But 30 minutes does not seem feasible for me.
It's not like you're going for a 30 minute jog where you just run without thinking. Don't you need to take time to "warm up" until the project you're doing starts being fun? And how does stopping just when you start enjoying yourself not kill your motivation? And don't you think about the project throughout your day job (instead of whatever problem you're supposed to be working on)?
Then a spare weekend will finally come around and I’ll have made enough progress that I want to spend an hour, or three, on the project.
The purpose is to make progress however you can. This is also why my other points are so important: work on something that will directly benefit you (scratch your own itch) and have a clear objective outlined.