Thanks everyone, for your good advice.
I was very tempted by https://docusaurus.io/, but I eventually chose the WP BSF knowledge base because it's ultra-light.
Machiel92 I tried bookstackapp and using it for a KB didn't work for me but that was 2 years ago.
@josedieguez I think the team of perfmatters use it too. I didn't choose it because it requires a separate WP install with another theme etc. Also, they are mostly compatible with WPML which is a really heavy/bloated translation plugin.
@kyzoeadmin I bought betterdocs LTD and tried for the past two years to use it on at least 12 projects but always failed and had to look for an alternative. Yesterday I tried again and dumped it again ;-) it's so bloated. You get locked into a three column layout. Also this plugin doesn't play well with Polylang pro.