What are the hard lessons of starting a side hustle while holding a full-time job here?

I ran a small home-based baking business alongside my 9-to-6 for two years, and the hard lessons weren't what I expected. First, check your employment contract for a moonlighting or conflict-of-interest clause, mine had one and I had to declare it to HR to be safe. Second, the admin will eat you alive, once you start earning you need to track it for IRAS because side income is taxable, and I nearly got caught out not setting aside money for the tax bill. Third and most brutal, the side hustle is easy when it's a hobby and hard the moment it becomes an obligation with paying customers who complain, that's when 70% of people quit. The ones who make it treat it like a real business from day one, separate bank account, proper costing, and they don't quit the day job until the side income is consistently covering their expenses.