[NosyHammock]

>So you're asking how to do it without a tool that was specifically designed for that purpose?

Yes, you are correct. This is a hobby project of mine, and one of the goal was to make old.reddit.com like UI without no JS at all. That's what I will do.

>I really don't get the hate for JS these days.

Unnecessary Javascript bloats websites, hogs system resources, enables surveillance, hinders accessibility that’s native to devices and clients, and introduces vulnerabilities. https://nojs.club/

Of course in this case using JS to hide/show comment tree wouldn't really be that bad, but works fairly well without it. Not many have complained.