I usually focus on existing rankings first. If a page is already getting impressions and sitting somewhere on page 1 or 2, there's often a lot of untapped potential there. A few updates to the content, better internal linking, or improving search intent alignment can lead to quick gains.
Once I've pushed those pages as far as reasonably possible, I start targeting new keywords to expand traffic. In my experience, improving what's already working tends to deliver faster results than starting from zero.