I debated creating this post, but it’s a useful tool you may not be aware of, and very easy to use.
Lighthouse is an open-source, automated tool for improving the quality of web pages. You can run it against any web page, public or requiring authentication. It has audits for performance, accessibility, progressive web apps, and more. -
It can do the basic checks for your onpage SEO, as well as loading times etc.
The SEO is limited to onpage, and is quite limited, but it’s a good place to start to ensure you have the basics done correctly. Some of the strengths of the tool are probably in the other categories - performance, accessability, and best practises.