SOOOOOO after finally setting up my kbin dev environment I went ahead and wrote some code to add support for EXACTLY this on the official kbin codebase!
GIF of it in action:
demo
I have a PR up for it already here: https://codeberg.org/Kbin/kbin-core/pulls/167
Now just need @ernest 's approval and once it's merged you'll be able to do this on kbin without any modifications!!!
edit: clicking on a comment now also hides its content (other than the username), making it even more clear which threads are collapsed! (similar to apollo) – updated the demo gif