I did something sort of similar. There are limits to what you can do with a side project, and joining a company gets you access to other people with different skill sets from yours.

In my particular case after finding a good fit with a small startup I told the CEO to just pay me as little as he could reasonably justify and make up the rest in equity (which I was fully aware would likely be worthless). In a different framing, I was "spending" my missing salary by "hiring" some people to do the work I wouldn't want to do - pitching deals, forming business relationships, and negotiating contracts to get me the data that I actually wanted to work on.

But that's not the idea is it? The idea is to define the terms, and see what the bids are. Perhaps, if the bids are too low, the decision would be to 'pay oneself' (spend from savings) while working on said side project. But the point is simple - an attempt to find a union of two interests at a price that makes sense.