I’d regenerate the complete HTML page server-side whenever a comment changes, instead of only regenerating the comments section, to obviate the secondary loading via JS.
It doesn't. Browsers will happily render half an HTML page as it's streamed in. There's an interesting HTTP 203 on this. https://web.dev/shows/http-203/LLRig4s1_yA/ if you're interested.
https://simonhearne.com/2020/network-faster-than-cache/ has some discussion.