Contrast, targets, labels
Three fixes account for most of the measurable gain: sufficient text contrast, tap targets of at least 44px, and properly labelled form fields. All three help every user, not only assistive technology users.
Keyboard order is a UX audit in disguise
Tab through your checkout. If the order surprises you, it will confuse everyone — the DOM order that screen readers follow is the same order that reveals a muddled layout.