I am afraid to sound millennial but why not WordPress + Static Site Generator Plugin? I totally get it, this is a fun project to test some skills but if the concern was going live, I can go live with vanilla wordpress with their default theme in less than a minute.
Off the top of my head, no constant site-breaking WordPress updates to install, no constant WordPress/plugin vulnerabilities, not to mention much less CPU/storage needed to host the site. Further, since the site is generated to static files, you can host it on any server technology without any concern for whether your given "stack" is supported.
Because it's slow. Even the fastest wordpress static site plugin still requires the huge wordpress site to initialize. I have never seen a fast wordpress site. I mean sub 50ms loading times.
With a plain nginx route to index.html containing some fixed CSS,js files is so much faster.
I think you're misunderstanding what Wordpress's static site generator plugin does. It bakes the WP content in to static HTML files that the server (eg nginx) then serves to the user. Website visitors never hit any part of WP. It's only there for the admin and editor. You can serve the static files from a different server infrastructure (say, an S3 bucket) with a little work.
In other words, it does precisely what you're saying it should do.
You only have to maintain the Wordpress site unless you do fancy stuff on top of it. And on modern Wordpress that's quite minimal. The point of this approach is that people with Wordpress expertise can continue to manage the website and you get all the benefits of a static website.
My Wordpress sites have minimal plugins (hence minimal JS and CSS) and run via a VPS and using Litespeed Enterprise, and they're almost as fast as static sites - but with 100x the ease of use and power.
Heck, my Wordpress blogs are faster than when I had static sites on shared hosting.
3) you get complete control over what content is generated (i.e. can minimize CSS/JS size)
4) you get complete customization - you can make your own way to template things that is convenient for you, or auto-import a particular file format you like to keep for your notes on disk, etc
5) you don't have to learn how to use wordpress
(It's not to say I think using wordpress is bad - if you like it, use it!)
I have been burnt many times by wordpress. Updates that fail and hose everything, continual hacks because you can't update, then automated updates that worked until they didn't, obscure PHP errors from oddly configured hosts, dodgy plugins that people add that get them hacked even if they are updated to the newest version of wordpress.
It is great for non-technical people, but the risks are high.
I personally find that github pages is a better alternative now: most users can handle doing edits in github's web UI, and on commit the changes are auto-deployed to the live site and appear within 60 seconds or so. And it helps me to see their git history when they screw something up.
I have a series of content websites, and at first I used Jekyll and it was fast to load, but everything took 10x longer to do than just using WordPress.
I now use WordPress on a VPS and served with Litespeed Enterprise and the performance (read: load times and CWV/GMetrix scores) is very very good - better than static sites hosted on shared hosting, in-fact.