Programming

There's a case to be made, realistically speaking, that using a well-known framework or even a CMS like Wordpress means less complexity specific to your website to understand for the next person. FTP cough SFTP or Markdown/HTML is definitely not beyond non-technical people to understand and use, but sadly there could be some resistance nowadays I imagine.

I would look into static website generators. Sadly I'm not sure what is most reliable nowadays, but I would prioritize easy of use and installation, as speed is probably meaningless on your scale. Here's a random article.

Document document document.

Also, the nice thing about libraries, frameworks, and CMSs is that they already have documentation. If it seems like a pain to learn one of those, imagine how much worse it would be for someone to learn custom code that doesn’t have any resources on Google.

maybe a static site generator and set up some CMS?

Have you worked with WordPress before? You could create a theme for the sites to maintain consistency, while letting the client have CMS access to edit the content.