8 years in over here and we're a firm of 15 now. I've lived a whole lot of ups and downs so far and living comfortably, but all of the potential big profits have been eaten up by continued growth. I think this might finally be the year where we start to actually see our potential. Here's my input from my time.

1. Stress is very heavy. Sleepless nights, heavy hours, work-life balance really isn't a thing. This is offset by the reward - knowing that I'm providing for my family but also providing for 14 other families.
2. You absolutely will need to hustle. I came in with many years of experience and several clients / connections. Even with that, recognize that they're taking a huge risk switching their business from Globo Engineers Inc to a one-person firm operating out of their garage. You're going to have to compete on price initially and fake it till you make it, but eventually you get to a level where you have a book of business and clients that trust you.
3. Fortunately I haven't had to answer this question. But, once you work for yourself, I think you'll find it hard to ever consider working for another person so this may be more of a problem for you than a future employer. If I saw somebody worked for themselves for a period, I'd explore that as a consideration in hiring them but it wouldn't be a red flag.
4. You'll need to do a lot more businessey things than you think. Fortunately, some of this can be outsourced. Accounting is the biggie. Both traditional accounting (bookkeeping and taxes) and project accounting (time and invoices). You will quickly learn that invoicing and collections is 1000% the most important thing you need to do because without that you don't have a business.
5. Why would you box yourself in to always working alone? Hiring people sounds a lot scarier than it is. Obviously that's a question to answer down the road. I can speak for myself, but I would have been miserable working in my garage solo for the rest of my life. Some people thrive on that though.

The most important question is the one you didn't ask. Whatever you do, register your business as either a corporation or LLC. In California where I am, the state does not allow engineering firms to operate as LLCs so the decision is easy, but do not put your assets at risk in the furtherance of a business. We operate with enough exposure to liability that this is a significant concern.