If you want to improve your site and not change the structure or design of the site drastically, I advise you to use extensions. Now you can find here https://www.mageworx.com/ many different extensions that will really help you improve your site quickly and inexpensively. In addition, you can read how each extension works and choose exactly the extension you need.
Anyone can make a website with full functionality and beautiful design, even a beginner with modest knowledge of computers and everything connected with them. But keep in mind that the wrong choice of platform and poor quality content can turn into big problems. So when I created my website, I chose the WordPress platform and all the images convert png to jpg because this format is one of the popular raster image formats used for storing photographs and is easy to upload to a website
I think that you have made a mistake. First of all, you can make a website CEO friendly, you can make it more SEO, with S, not C. It requires some knowledge to make it more SEO friendly. You need to learn about SEO a little bit. It doesn't take a lot of time. Secondly, you can make your website SEO friendly when you have an example. For example, wordpress theme for a multivendor marketplace is the perfect example of an SEO-friendly site. It is just perfect. It doesn't require any more modification. It is just perfectly made.