It seems odd that I hear people talking about getting 0.5% equity joining as the first engineer of a startup. Chances are you are getting paid at least 25% less than you could elsewhere, and let's assume you're making $100k/y. Trading $25,000 of salary for 0.5% suggests the company is worth $5 million. That seems incredibly unlikely for startups hiring their first engineer, and also doesn't account for the tremendous increase of working hours and the greatly diminished job security.
This is never a deal I would take, even as a junior developer not making six figures.
That's a tough pill to swallow but it's a good thing you realized it and are doing so now, rather than after you took the job as first engineer. Your insight is 100% correct. You, as the first engineer, are taking on a huge amount of the risk of the startup and the work but you aren't getting the upside like the founders are.