Yes. The idea of giving whole integer percentage points of equity to someone who can't vest makes me nauseous.
There's three problems with your plan:
First, if the high-end of what you'd think of asking is 18%, then it seems likely that even the low end of your ask is in the founder/first-employee range. You're talking about numbers that hired CEOs get.
Second, the stake you're looking to take in these companies practically guarantees conflicts down the road; you're asking for so much that you're going to have to take an intense interest in valuation and dilution concerns. Who involves a contractor in things like that? Who gives a contractor a veto on funding or acquisition? Who funds a company that values 8% of their company so low?
Third, despite having risked virtually none of your own capital (again: evenings and weekends, divided among multiple companies), you're asking for a share comparable to what early employees who dedicate themselves to the company and take reduced salaries get; those employees, by the way, will all vest, unlike you.
I don't think this is a workable plan.
I think there may have been an interesting conversation in the 0.1-0.5% range, although my plan was to explain why it's dumb for both sides to give 0.1-0.5% of a company to someone as direct compensation for a transactional service, and how you could have made more money and made startups more happy by coming up with a clever deferred comp scheme pegged to equity valuations.
But at 8-18%, I'm not sure where to go with this.