You're ignoring a powerful aspect here: Founders are cash-poor, but equity-rich (almost infinitely so).
I don't know whether you have enough money to cover your costs (I assume so, you sound quite successful), but I think it would be interesting to seek out promising founders and take a little bit of equity, instead of another "bunch of money"(tm). That increase in money is not going to significantly change your life, getting an early in at a startup might though. And in addition to that, you'll probably enjoy building that umpteenth MVP a little more if you have the feeling that the quality of your work has a potential stake in your future.
There's of course a lot of my making assumptions here, but I think it's an interesting thought to consider.
This is where my other rule comes in: "I am either a freelancer or a cofounder".
To be your cofounder you must also convince me that this project is something I want to spend the next several years of my life on and it must be great enough to convince me to drop other projects and start working on this full time. I don't believe in part-time cofounders, that way lies burnout and failed projects.
And despite all that, equity still isn't tasty so I will need to earn at least some of my normal rate anyway.
Would you ever consider being a cofounder before the MVC is built?
I guess you're saying that you'll only build MVCs for cash, but then will consider joining as a cofounder afterward if the project looks promising and you can work out a fair equity agreement?
I guess it depends. I usually have enough of my own pre-MVP projects that would love some attention and often some that are MVP's and would also require attention.
But building a MVP together before jumping into a full blown partnership is also rather important in terms of finding whether there's a general fit in working together, you're actually able to deliver on the non-technical aspects of the project and so on ... MVP's are also incredibly good at confirming assumptions about the market.
This is the part about equity that is most likely to bite you in the ass if you receive equity but ultimately do not control the allocation of it. (Common for early employee or founders on the technical side of things).
Day 1: Awesome, I own 10% of this company. If it sells for millions I will make a significant amount of money!
Day 630: Due to the "almost infinite" nature of equity and dilution, I now own 0.05% of the company. If it sells for millions I will be able to buy a cup of coffee!
If you go for this sort of deal you should always have some anti-dilution protection. Otherwise as is pointed out the equity is essentially worthless. You can use some sort of full ratchet clause or ensure the majority shareholder can not issue more shares (though some special resolution requirement). You'll need a lawyer...
I don't know whether you have enough money to cover your costs (I assume so, you sound quite successful), but I think it would be interesting to seek out promising founders and take a little bit of equity, instead of another "bunch of money"(tm). That increase in money is not going to significantly change your life, getting an early in at a startup might though. And in addition to that, you'll probably enjoy building that umpteenth MVP a little more if you have the feeling that the quality of your work has a potential stake in your future.
There's of course a lot of my making assumptions here, but I think it's an interesting thought to consider.