First, calibrate: promotions is not spam. Mail there still gets seen, and heavily designed marketing mail belongs there by Gmail's definition. You can influence the sorting; you can't override it.
What pushes mail toward the primary tab: plain-text formatting, few or no links and images, a real person's from-name, individual-sounding copy, and recipients who reply or drag your mail to primary (each drag teaches Gmail). What guarantees promotions: image-heavy templates, coupon vocabulary, many links, tracking-heavy footers.
Strategic answer: make the sorting irrelevant. A useful email from a person, sent to a list that wants it, gets opened in either tab. And replies (the strongest primary-tab signal) come from usefulness, not tab-hacking.