When YOU make Static Site (with SSG), what additional APIs do YOU eventually have to use and how? (third-party or self-made)

So, static site generators are lies.

This is a bit misleading, or possibly the result of someone else writing something misleading. Static site generators are incredible at the thing the set out to accomplish, which is quickly responding to client browser requests with static files. On-the-fly rendering cannot compete with the speed of static content delivery, which is why CMS's like WordPress lean heavily into caching plugins for scalability.

Now to the actual problems:

• Comment solutions for static sites exist, and they generally just load up a JS library after the page has loaded. Disqus is one fairly popular option, Commento comes up in a quick search, and there are several others.
• Search engine solutions also exist, but the correct one depends on your needs. If your site is small enough, you could build the search function in JS and build the search cache when you rebuild your site. A serverless function is a common way to handle this, like what you're doing.
• Email and subscribe functions are also often handled with serverless functions. Netlify has Netlify Forms which would make the setup fairly trivial, although isn't free once you hit a certain number of responses.

As for security, it's a bit of a complex question. What type of security requirements do you have for a search engine? It seems that this would be public-facing, just like your site. And the same question exists for the email/subscription functionality. Perhaps include some sort of robot detection if you are concerned about spam, but what other security issues are a concern if it is a public-facing form? The connections for both of these are simply public API endpoints.

Static sites are fantastic at serving infrequently changing data to browsers. The problems that you called out do not fall into that category, and they all work with a back-end to accomplish their tasks. The only reason that a CMS is "easier" in this regard is because you already have a back-end running to serve your content. The thing is, it is trivially easy to serve a static site. The server requirements are much less than that of a CMS. The speed and ease-of-hosting come with trade-offs, as you have found, but a CMS has a different set of trade-offs such as speed, cost, and infrastructure complexity.

Hopefully this helped, I admittedly haven't finished my cup of coffee yet :)

SSR is slow