Black hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines guidelines.

Toxic backlinks are the unnatural links that can harm the search rankings of your website. I would suggest to make a list of those links and disavow them through your search console or if you have control over those backlinks just delete them from those websites.

Here is the complete guide that you must read before you proceed - https://support.google.com/webmasters/answer/2648487?hl=en

It's hard to know how many keywords you should use on a page for the best SEO benefit. Because it always depends on the length of the content but it should not be used more than 5 times. And the relevant keyword should b used if the keyword is inappropriate according to the content than it should not be used.

Here are some of the important factors to be considered while choosing the keywords

1. Search Volume
2. Keyword Competition
3. Number of Search Results
4. Relevance of a keyword
5. No keyword Stuffing1.

Here is the thing you can do,

Use Crawlable Anchor Links:-

Your root page URL should be like - example.com/category 2nd page URL Should be like - example.com/category?page2 so on the the remaining URLs

For search engines to efficiently crawl paginated pages, the site must have anchor links with href attributes to these paginated URLs.Be sure your site uses <a href=”your-paginated-url-here”> for internal linking to paginated pages. Don’t loaded paginated anchor links or href attribute via JavaScript. Additionally, you should indicate the relationship between component URLs in a paginated series with rel=”next” and rel=”prev” attributes.

Complement the rel=”next” / “prev” with a self-referencing rel=”canonical” link. So /category?page=4 should rel=”canonical” to /category?page=4. This is appropriate as pagination changes the page content and so is the master copy of that page.

If the URL has additional parameters, include these in the rel=”prev” / “next” links, but don’t include them in the rel=”canonical”.

For example:

<link rel="next" href="https://www.example.com/category?page=2&amp;order=newest&quot; />

<link rel="canonical" href="https://www.example.com/category?page=2&quot; />

Doing so will indicate a clear relationship between the pages and prevent the potential of duplicate content.

BERT, which stands for Bidirectional Encoder Representations from Transformers, is a neural network-based technique for natural language processing pre-training. In plain English, it can be used to help Google better discern the context of words in search queries.

Focusing on the right keywords has a huge impact on growing your traffic. The added benefit of keyword targeting is that this content will bring you visitors who are more likely to convert to leads and sales.

Writing content for SEO is a lot more than sprinkling a few keywords throughout your text. Google will certainly not appreciate it very much. Not only will you fail to get your site ranked highly by it, but you also risk getting penalized.

I am totally agree with @shane's answer. Google doesn't affect those extra trailing slashes and won't make any difference in your ranking until they are hyperlinked correctly. Basically anchor text allow users to navigate a website. They help establish information hierarchy for the given website. They help spread link juice (ranking power) around websites.

Well, if you want to remove something from your own website than there is an option given in the Google search console. Just follow the following steps

1. Open your search console Property.
2. Open removal section from your left and click to the "New Request"
3. There are two options available when you click to the "New Request" 1st is Temporarily remove URL and 2nd one is Clear cached URL. Please make sure you will read the following guide before you proceed. https://support.google.com/webmasters/answer/9689846

What if all the checklists are correct with the website but it still not performing?