Here in EU, if not stated in your work agreement, it's pretty common people work full time job and also as a self-employed contractor for other companies.
So when I'm finished with my work, HO of course, I just work on my "contractor" projects.
Honestly, I wouldn't sign a full time contract banning me from other work.
And if you have enough customers, you just drop full time job. And just pay social security and health insurance, which you must pay by law anyway.
And specially in my country, it's even more ridiculous that as self-employed you pay lower taxes than full time employees, which truth to be told are ridiculously high. Nearly 40% of your salary.
Freelancing as a side hustle may be forbidden if your employer refuses
And it makes sense to pay more taxes since you also have more social benefits (paid leaves, retirement money and unemployment money), nothing is free