- Above the fold works well for adsense, for reducing core web vitals scores(cumulative layout shift) and for reducing traffic in the process. The later 2 aren't good so use wisely.
Question, how do you know your top ad is not just under the header above all content? Do you have auto ads turned off? If not adsense will toss ads up there and though you can block up to 10 ad slots the one just under the header is near imposible to find and block in your ad settings preview. Sometimes you can see it sneaking up there in the pagespeed screenshot.
Also, about core web vitals, the score doesn't represent any one page. Your pages are grouped and averaged so reports to any page in the group affects all pages in the group, including the one you looked up. You can find grouped pages in your search console. Consider removing ad code from pages in the group that get little traffic, the least they can do is improve overall site metrics. YMMV.
<?php if (is_single('50')) echo ' AUTO ADS CODE HERE ';?>
If you place that in your header template file for your wordpress theme it will insert your ad code only on post 50. You can ad multiple pages by adding a coma after the 50 and every other post number you include. You can also add multiple conditions like is_page, is_home etc. You can find the post number in your wordpress dashboard, in the posts section click view all and find the post you want to monetize, the post number is part of the link to the post, hover over it to see the number.
In your adsense settings turn off in-page ads and turn on the anchor ads(but not the widescreen). Next create a vertical display ad and place it in sidebar 2. In wordpress you can create multiple sidebars so use sidebar 1 sitewide and sidebar 2 only on pages you want ads in, like post 50. Use the html widget to hold the code.
The result, ads only on desired pages with an above the fold ad on desktop(in sidebar) and an above the fold ad in mobile(anchor ad). Though above the fold both these ads have little core web vitals impact other than how slow Google is. The anchor ad only appears when a scroll is detected and the sidebar ad code is technicaly at the bottom of the main content. It doesn't block.
Manually place more ads on that post via manual ad units if you wish, the end result is most pages are fast with no ad code and the most trafficked posts are monetized.
It's the best tradeoff you'll find in the war between pagespeed and adsense, IMO. Good luck!
Edit - if your most trafficked page has very low CPMs make sure you A/B test some lesser trafficked pages. If a page pays 10x more but only gets 10% of the traffic it pays the same as the more popular page. Slow code affects fewer people overall, a good thing.
/ends ramble /hopes he gave you some ideas
