You seem very happy with your current situation. It sounds like you'd be taking a cut in your pay in exchange for much more work. There's a small chance of a large payout if they get bought out, but this is assuming the startup doesn't fail (as most do) and that you stay long enough to be there when it happens.

If they aren't willing to match your current salary and give you a much bigger slice of equity, I would decline the offer.

Do you believe in the product/company? Does your equity dilute? Do you believe in the founders?

Unfortunately, the odds of your start-up succeeding are very slim. Not to say that it's impossible but I wouldn't bank on it. I worked at as a founding engineering with two sr directors that left a prior employer. it was fun for a couple years but then I moved to the bay for big tech. it's been close to 14 years now and I'll retire by the end of the year. all this to say, if I was you I would maximize my income and saving at this young phase of your career. big tech is still an excellent place to build your network. at some point as you climb the ladder the corp culture may leave you dissatisfied and a start-up may be more attractive but hopefully at that point you'll be financially independent

It really depends on your personal perspective on life. Consider the two statement and which one aligns more with yourself:

1. "It's better to regret something that you have done instead of something that you haven't done."

2. "I want to retire comfortably with a high level of confidence."

People in my life always advised me to follow the highest paying job. I'm not sure that was the right advice given my circumstances. It's really up to you.

ignore 3% equity; will you deliver the ask, working 7 days for 160k, and for a questionable title of founding engineer? you'll get no technical guidance from the founders (who get to keep +80%), and only get requests to pivot to current flavor of the week; ask for 10% if you still want it and see what happens; if they say no, you dodged the bullet early;

250K is about $1000 a day in cash and your present employer is likely to be putting about another 250K into your non-cash benefits at the same time.

I would take advantage of your present (rare) work-life balance to cast some of it to the wind and work for these guys as a consultant without resigning from your employer, as long as it is not a conflict of interest.

Even help them find someone in their price range with the confidence that you can still back them up as a consultant going forward.

You hope non-technical founders are good deal-makers, but underhanded dealing is all too common as well.

So deal. One of the sure ways to find out how honest and generous someone is. I would accept less than the 160K on the table this year but with equity in proportion regardless of being "only" a consultant who might end up building the whole thing anyway. But it would have to be $1000 per day and if it turned out to be 80 days the next year it only cost them 80K with no benefits. What a bargain so there's no excuse not to stick with a calculated 1.5% proportional equity in this phase of the lottery. And keep on going :)

That would be about my final settlement point, where you start and how you negotiate down from there you're going to have to play by ear.

The next best thing to being a founder is to be able to write your own contract regardless. You already have an employer-employee relationship, that just doesn't sound like an ideal kind of relationship when it's going to be functionally a 3-person startup of "equals" the minute you start building.

Have fun :)