to be pending, to be due, to be outstanding (Mainly for bills but it also works for expected replies. Sounds formal.)
to bear something (Pretty much ONLY used in the past tense, for extreme situations you "stood through". NOT in the general sense of "bearing" as an ongoing process)
to not be able to stand someone ("jemanden nicht ausstehen können" - pretty much only used in this phrasing and only in the sense of not liking someone. The general word for "to be able to stand/bear" is "aushalten".)
So the go-to word for “to stand smth/to bear/to put up with” is aushalten, but if i want to say “i can’t stand him” it would be more idiomatic to use “ich kann ihn nicht ausstehen”?
Yes, because the focus there is that you don’t like him. “aushalten” is only about the idea of bearing hardship or difficult circumstances without “breaking”.