"I borrowed 30,000 from my IRA and returned 20,000 within 60 days. How do I enter form 5498, I should not pay tax on the 20,000"

You don't enter the 5498.  You only enter the 1099R.  One of the questions will ask what you did with it.  You select you rolled over part of it, even if you put it back into the same account.

Was that your only such payback in 2018?

Semantics...but you cannot borrow from an IRA, as that is prohibited by law.  As indicated above, you took a distribution, which you then rolled back.