Pardons don't cover impeachment.

The house will most likely impeach Trump again next week. Could the Senate try him after he's out of office? There are differing opinions.

https://www.nbcnews.com/politics/donald-trump/can-trump-be-tried-senate-impeachment-charges-even-after-he-n1253544

I believe, most likely he can be tried for the following reason: Even though he would be beyond making any governmental decisions, he would still gain all the benefits of having been President, including a healthy pension and a life-time of secret service protection. As far as I know a conviction and following judgement could result in him being barred from ever holding Federal office again and possibly strip him of his "retirement perks." Stripping him of secret service protection sounds cruel, but if he's anywhere near as rich as he claims he could pay for his own protection.