Then go look at who's above you now and what the result layout looks like. A jump from 2 to 9 in nine days with no site change is usually either a competitor refresh or the SERP itself changing shape.
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Fixes information architecture on sites that grew to 40,000 pages without anyone noticing.
Start by confirming it's a ranking loss and not a demand loss, because they look identical in a sessions chart. Pull impressions and average position for that exact query in Search Console and compare year over year. If impressions held and clicks fell, you slipped in the ranking or lost a feature you used to hold. If impressions fell too, fewer people searched, and June is a real seasonal trough for a lot of topics. I've watched three people rewrite a perfectly good page because of a seasonal dip.
Bump products per page to 48 and add curated collection pages for the ways people actually shop. Depth 9 to depth 3 is a bigger lever than anything you can do to the product copy.
You've basically answered it in the last sentence, but the technical half people miss is internal linking depth. Pull a crawl and check how many clicks from the homepage each unindexed URL is. On most migrated shops the indexed ones are the ones linked from a category page in the main nav and the rest are only reachable through pagination or a sitemap, which is the weakest possible signal. Get every product within three clicks and linked from at least one hand-built page and a chunk of that 300 will start moving within a few weeks.
One thing that isn't time: sales. Ask whoever sells whether they send prospects a link to any of those 40 pages. I've consolidated away a page with 11 sessions a month that turned out to be the page the sales team pasted into every proposal. Traffic isn't the only reason a URL exists.
Flip-flopping is the classic signature, yes. Before merging, check the query data for each URL separately. If the 2024 post gets 80% of its clicks from a different set of long-tail queries, merging throws that away for a head term you might still not win. The test I use: if I removed one page entirely, would I lose any query I currently rank for? If no, merge. If yes, differentiate hard and stop competing.
Then retitle the 2024 one so the head term isn't in the title or H1, and link from it to the guide with the head term as anchor. You're picking a winner rather than hoping one emerges.
Twelve posts is also just not very many. A site with 12 pages has almost no internal linking to work with and nothing that looks like coverage of a topic. I wouldn't expect much before 30 or so posts in one tight subject area.