I would keep it pretty simple. Royalties can be complicated and so many other factors involved that can affect sales that you have no hand in. If you believe in the company overall, equity can be OK to add to the equation, but it’s more a long term play (equity only makes you money if they sell or you sell back your shares). Equity only makes sense if there is already a valuation and investors on board that have paid in at set valuation.
It shouldn’t be that hard to figure out your average hourly involvement per month and a rate that you are comfortable with. Set a monthly retainer either with open hours or a capped hourly set with overage on agreement and a flexible buffer under/over that can roll over month to month.
I’ve done a similar arrangement and it worked well. Just be sure to get a good deposit of several months in advance and good payment terms so you don’t end up working and find there’s no money coming in for time worked.
Number of hours and rate is up to you. You should be able to base it on your consulting work for over the past year in relation to what the new job expects.
Either way, be sure the job description is solid if on retainer, you don’t want to be working full time burning hours doing something the intern should be doing just because they figure you are “free”.
Good luck,
R