It's really easy to pull $300k/y as an SRE (but that does amount to about $100/hr equivalent in SMB, assuming realistic >40h workweeks and the tax advantages in SMB, even less if you have to pay for services you enjoy and can't find time for due to FTE - nannies or even changing your own oil).
It is extremely hard to keep that $100/hr and a sliding scale - one week you work 50h, the other 30h, then you take a month or three off at $5h/week.
Mine (and I don't regret these) is running midsize PV plants. Slightly bigger than residential (hey, does OP want solar PV on the rooftop? I have a bowling alley as a client, they are a great match), but again not large enough to attract a scaled business.
I approximately put in 5h on each a year after the initial setup (around 100hr each), for $10k/y in profit from each.