I suppose for side-hustles it may help doing something that's somewhat removed from your day job. Rental property comes to mind. It's fairly rote, it's social, you can put in as much/little elbow grease in it as you like (barring the rare emergency), but it does generate modest income, it's usually more than just a "hobby".
A friend, who doesn't work in finance, is really into the stock market, spends a lot of time on researching companies. That's a kind of side-hustle too I suppose.
Full time work is a misnomer in common use. If companies wanted to control how I disposed of all my hours, there'd be a much higher price involved.
Then HR changed the promotion policy. You could only get promoted if you had 6 people under you. My manager immediately hired three more people to "stay" a manager. Then, he encouraged each of us to hire people under us, the wordload was mostly the same, we just created a lot more processes, controls, documentation, PMO, etc. But not enough to consume all the time the now-gigantic team had. While incredibly wasteful for the company and our owner (the US government) we had so much free time we were doing contract gigs during office hours. Two guys had full-on startups in play.
In the past I've done a lot of side work much to my full time jobs benefit. Not so much in the last couple of years I've changed around some priorities. It was a bunch of fun though you should give it a try!
I don't think there's anything unethical about it at all as long as it's not competing with or using information from your employer.
Modulo burnout and things like that, of course.