Well, I'll describe basic microblogging actions as:
Reply - Extend a conversation, create a new twt in reply to another
Boost, Retweet, Repost - Silently show a twt to your followers (maybe to share some interesting URL to follow based on that twt)
Quote - Start a new conversation based on a previous twt (debatable, for instance Masto doesn't have this one, has to be done manually)

To go back a few steps I was checking this doc:
https://dev.twtxt.net/doc/twtsubjectextension.html

Reply is already covered there. Allows to tag someone with @nick1 which I think is good enough to start following that URL to see the whole conv.
For quoting, I like your idea of using '>'
It could be something like
{{{0}}}

Finally for boosting, there's no syntax to do that.

Since I was using
[lang=en]
to define a twt in a language different than the default one, I was thinking on something similar.

Perhaps
{{{1}}}

Meaning, "hey! see that twt with hash # from that URL"