The OP's language makes it seem like the relationship is already strained and he is isolated from the founders ("half-baked", "shit money", "feeling helpless") while still wanting to continue with the company. This negotiation is wrapped up in two things: 1) the financial issues and 2) the OP's value to the company. I think point 2 is the trickier part. Others have already suggested a few ways that the financial issues could be resolved, but he also has to get his relationship with the founders into a better state. If the relationship stays strained, it's only a matter of time before the next issue sets this whole process off again. Especially when we're feeling isolated, rearranging our own mental state often starts the process of claiming the power to fix the relationship.
Let's say they negotiate and his equity stake is cut back to 8.5% while the others give up 5.25% each. His narrative of the negotiation could be, "They tried to screw me again and I barely held on" or it could be "They gave up more money, I gave up a bigger proportion." Even though the finances don't change, the first response maintains the status quo while second response starts the process of reclaiming the relationship.
(Sorry for the male pronouns if you're not a male, OP. Writing he/she is annoying and I'm lazy.)
Let's say they negotiate and his equity stake is cut back to 8.5% while the others give up 5.25% each. His narrative of the negotiation could be, "They tried to screw me again and I barely held on" or it could be "They gave up more money, I gave up a bigger proportion." Even though the finances don't change, the first response maintains the status quo while second response starts the process of reclaiming the relationship.
(Sorry for the male pronouns if you're not a male, OP. Writing he/she is annoying and I'm lazy.)