I'm trying to bootstrap a consultancy/freelancer business.
For now, I have a possible situation where I can work full time and then also work part-time probably on a contractor basis (not sure) for another company.
What do I need to know about this? What are the legal considerations? How do taxes work? Lifestyle stuff I should worry about?
Do you have any war stories about this kind of thing?
Taxes are usually done after the fact. I.e you get a 1099-MISC from each person who has paid you, and you are liable for that.
Speaking of payment, learn to be a squeaky wheel about it straight off the bat. Freelancing is notoriously inconsistent, expect 60 day swings between receiving payment. DONT BILL BY THE HOUR. There's plenty of reasons why online.
Lifestyle-wise, figure out a way to be productive. You have to enforce accountability in yourself, you don't have a boss or a team counting on you. It doesn't matter when you work, you don't need to show up 9-5, you just have to get your shit done.