For your internal analysis, a multilingual website could be intriguing. Both options you're considering are viable; neither is incorrect. With a multilingual site, you can evaluate directories and treat them as specific country regions. Moreover, it simplifies tasks when you use tools like https://www.screamingfrog.co.uk/seo-spider/ or quick check tools such as https://www.seoagentur.de/kostenloser-seo-checker/ to analyze the site. This setup might streamline managing your SEO across different languages and regions.

You have rankings for the 3d chalk art page. They are not in the visible range, but they also have their right to exist, and I would focus on optimizing specific topics on subpages further. Have you integrated alt texts, used more long-term terms, or done anything with Google Business? What about backlinks?

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You have to think of your intention for the Google SERPs. Are those pages optimized for the same specific keywords or long term phrase. If yes, take a close look at the performance and ignore canonical Tag or robots settings in the first steps. When you see the ranking performance between the subfolder you will notice which strategy will fit for you. Optimizing a big website for several duplicates is not really helpful but not completely wrong.

@nkennett I can understand you very well. But take into account that new website usually can take 6 months or longer (depends on the competition) to generate significant increase in traffic. I needed 6-8 months for my homepage until the first top 10 rankings were tracked. SEO Is investment. Rankings are not coming in a few days or weeks. It needs time and during this time you put much effort into this project and never give up.

Without seeing your website with the structure and technical settings it is rather hard to say which reasons are responsible. Usually making request to Bing Webmastertools should index your pages easily.

It is a content issue. I have worked a lot with H2 and H3 headlines and see many SERP results with sitelinks with Headlines keywords. If you optimize a content with different parts and work with tables of contents, Google can recognize the structure understand your content and evaluate it. Some parts will be shown in the featured snippets as well or in FAQs. To say that they don't have any impact is wrong.

You can make complete Crawl of your webpage with Screaming Frog. If the number of pages is not large. You can check the internal Links and export it. With Pivot tables you can identify all pages which link to a specific page.