My amateur story as SEO-enthusiast- what i am doing wrong?

What about the UI/UX of the page?

Site structure is essential for ecommerce, make sure you have proper hierarchy and well organised listings.
For onpage audits use Surfer SEO, you might have too much content or just not enough unique content. For offpage use AHREFS and between these 2 sites you should smash it.

BTW, on backlinks he may be using PBNs which don't get picked up by tools like AHREFS.

from what information you provided I get the following feeling (could be wrong but need to see the actual project to further analyse):
1. Backlinks: You have a lot of backlinks for the DA your site has. Always remember, that its not always better to have a big amount of links, quality is essential. For example, I would prefer 1 content link from washington post over 10 blog post links of medium sites. Some sites act as core sites. Links from these carry much more value than usual links. Do not go for cheap link building services and web 2.0 links. These can be used for parasite projects or other blackhat-ish projects.
2. Content: Having large articles is not always best. yes, it raises the chance of including more entities that google can pick up, but if you did not plan the articles well its pure luck if they are included. Check the top ranking articles you want to compete with and cover everything the top 3 cover + make sure to check meronyms and hyponyms. Try to cover these (if it makes sense).

Try to become an authority in your niche by creating well written, indepth articles. Also make sure to write the articles in an easy to understand way for googles crawlers. The cheaper it is for google to understand your content the higher the chances to rank. How Koray Tugberg put it: "It must be cheaper to rank you than not to rank you".

The links that you have, are they niche related. Or just spammy links?

The age of the page could possibly be a factor?

Raditashev,

If you have analytics installed, what is the Session Duration for that page? You might have to go back when it was ranked in the #8 position. How long did visitors stay on that page, and did they interact with it at all? (This data can be found in GA4, if you have that setup for your site.)

Once your page is considered relevant for the search query, the next hurdle is to send the correct user engagement signals to Google. In effect, your visitors are voting with their time/clicks/scroll and tell Google if the page is a good match or not for that search query.

A lot of this information came out with the Google antitrust case.

This article links to, and goes over the court document. The key related concept here is NavBoost.

"Navboost is trained on click data on queries over the past 13 months. Prior to 2017, Navboost was trained on the past 18 months of data."

So, user behavior becomes a primary ranking factor, with this system.

How about installing Microsoft Clarity on that page, and watch how users interact with the page. See if there are ways you can send more positive user signals to Google, and over time, that should correlate with a boost in your ranking, relative to the signals those competing pages are sending to Google.

Ranking isn't just about perfect scores and backlinks. User engagement and content relevance play huge roles. Your competitor might be hitting the user intent better, even with less optimized content.

oh have a lot of backlinks for the da your site has. quality is essential.

The challenge may lie in content alignment and search intent. While your site performs better in technical metrics, your competitor's content might be more closely aligned with what users are looking for, even if it's minimal. Consider revisiting your keyword strategy, ensuring your content matches search intent, and focusing on providing unique, valuable insights that truly engage your audience.