FWIW I thought they were well implemented, though unique.

I think the difficulty for some readers (myself included) is that they are non-standard/different from how every other web page works.

I actually quite liked them, kind of like an extended aside.

Having a page consists of a series of "tabs" is not a very good pattern because it's not at all intuitive what is going to change (and it's only going to be part of the page) when you actually click it.

Would rather just have a table of contents with items and sub-items.

Table of contents (made of links to sections) at the top?

I experimented a bit with a ToC wikipedia-style but the results didn't fit very well with the rest of the design. I'll try to allocate more time in the future to focus on the design.

Oh, many years ago I wrote a script to automatically generate a Wikipedia style ToC based on the semantic structure of an article.

I will find it when I get home and reply to this comment.

Ah don't worry about that, I use Hugo which has a ToC generator. My problem is regarding the style (CSS) which I need to spend more time to make it blend better with the rest of the site. Thanks anyway!