I'd think about "what real offer would I need to up and leave?" and then spend a certain portion of your week pursuing that kind of offer, maybe start by finding a friendly recruiter to take the strain. You can let your friend know that offer when you get it, and what discount you'd take to continue to work with them (if any).
You might end up feeling like a crummy time for tech work (in the UK at least), and a good time to be settling for a lower salary doing interesting work, or you might find a crazy well-paid niche for your expertise that you never knew existed.
I hear what you're saying that you feel your expertise with their code & domain puts your in a position of maximum leverage, but I doubt your friendly founders see it that way. If I were advising them, I'd be urging them to consider nobody is irreplaceable (even if they end up paying someone new higher than they ever considered paying you). I'm sure your 1-2 year estimate for a run-in is pessimistic.
Would you consider a high day rate for contracting and let the employment and equity slide? If the company sold for £100m tomorrow and you got £200k, that sounds nice - but is it likely compared to the opportunity cost of working towards a real career ladder? Is it possible that they could dilute that 0.2% away? idk man, it doesn't seem like something I'd consider compared to salary.
Also - as others have said - don't bring your other income or into your salary negotiation (or let your employer do so, if they're aware of it). It's not relevant.
Sorry, not a complete strategy, but good luck with it.