Not sure if you misspoke or are just unaware of it, but Hack is one of the prepatched nerd fonts: https://github.com/ryanoasis/nerd-fonts/tree/master/patched-fonts/Hack. Also, for any fonts that aren’t prepatched, there’s a patcher in that repo to make any font a nerd font.
No, practically speaking the domain name should have no effect on access time. DNS has so many layers of caching that as long as SOMEONE has accessed the website nearby (including you), the domain lookup will be local and therefore fast.
Anyway, DNS lookup times, even slow ones, are still not going to be noticable to the end use originally.
Do you mean admonitions? E.g. info, warning, etc? There’s precedent for that in commonly-used open source implementations, e.g. obsidian.md (which uses the same syntax, and started before). What semantics does it break? It’s designed to read well in plaintext and render nicely even if used in a renderer that doesn’t support admonitions, e.g.
As opposed to other common markdownish implementations that use nonsensical plaintext which renders poorly in alternative renderers. Here’s a discussion on the topic in the CommonMark forums.