Feed quality or asset quality - which one actually moves PMax?

The honest answer is the one you already wrote, but here is how to decide it rather than guess. Feed first, in almost every case, and the reason is sequencing rather than importance. The feed determines eligibility - which searches you can appear for at all. Assets determine conversion of the impressions you win. Improving the second while the first is broken means making a better case to a smaller room. There is a test that settles it for your specific account. Look at how much of your spend is going to shopping-style placements versus everything else. If it is dominated by shopping, the feed is the campaign, and creative is decoration. If a real share is going to display, video and discovery, the creative is doing more work than your instinct credits and the agency has a case. One more thing that is cheap and is neither project: fix titles before anything else. Title is the highest-leverage field in the feed by a distance, and rewriting them costs a spreadsheet rather than a photoshoot. Do that first and you may find you can afford to wait a quarter on the rest.