Well there are too many things to list about improving ranking as I can write a book on the subject but generally, I see that old sites with old coded designs simply do not rank well anymore and generally anything older then 4 years barely gets any traffic at all, if the design of your site is very old it for example still uses old code and coding methodologies and is not keeping up with change such as the formats of your images and speed of your site the search engines can see these software versions you are using and it also knows your site design from history and it likes to see activity and change in your website as well as occasionally new designs. Most likely you need a modern coded design. Honestly, there isn’t much your likely to be able to do with an outdated design as the search engines record quality and ux scores on the site and analytics and the only way to reset those is a significant change in the code such as a new design. It’s like this if you take an old table-based website still using old html from 1999 or even or even something from 2014 and put it up against new websites it simply will not even be given a chance in competitive searches because the support for the software and code it uses is outdated and typically will not render well and deliver a poor user experience. If the design of your site is old that is likely your cause if not its likely other issues that need to be addressed with the site.
Login to your Matamo panel and you can find it it in Behaviour -> pages it will show you what pages people have been on . Count how many people were on your cart page vs how many checked out.