I've been meaning to look into KaTex. Could you elaborate on why you switched away from it? KaTeX appears to support server-side rendering already, in the sense that it generates plain HTML.
I started using KaTeX sometime after 2015 because it promised to be fast (the fastest! [1]). I had to change the representation of a bunch of expressions because KaTeX didn't support some environments, whilst MathJax did. It was a trade-off I was willing to accept at the time.
Many years later, I started writing a personal static-site generator. I wanted comparatively lightweight pages, so rendering server-side was an option. I re-evaluated MathJax vs KaTeX again and this time I leaned towards MathJax, as speed was no longer an issue for me. It looks like KaTeX has broader support now [2].