You call it your product? Did you meet with the customers? Did you build it in a complete vacuum? Is it completely unrelated to anything anyone else in the company was building? Did you pay your own salary? Probably not in all cases. You probably benefited from the context of the company that you're in. It's the company's product, not yours.

"Most importantly: how do I best stand up for my own interests here, without being the asshole that ruins the deal for everyone?"

Simple, don't try to ruin it for everyone as your negotiating tactic. Why would you want to do that? And absolutely no one expects you to be a slave or to accept offers sight unseen. That's ludicrous and weird if you expect that is what people are thinking.

If you don't like the offer, don't take it. If you like it, great. It is as simple as that. That being said, if you think that's a possibility I seriously suggest you have a great back up plan ready (another job offer, lots off ideas you want to pursue on your own and money in the bank, etc).

One thing you need to remember, is that while you are valuable as an individual contributor, the founders are valuable for being able to attract and lead individual contributors of your calibre. Believe it or not, the latter is far more valuable than the former.

This is why when you take a job, you really want to take a job working for people you really respect and admire, because they get a portion of credit for everyone that works for them. And there is nothing more soul destroying than someone getting credit for something you've done that they don't really deserve..