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.

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.

maybe a static site generator and set up some CMS?