Trying to remember when he supported Jan 6? I think you're confusing Blunt with Hawley. Whether or not you like him, he's not retiring because he hates being a Senator. He's retiring because he clashed with trump and fell out of favor with Missouri Republicans. Blunt was one of the people denouncing Jan 6 as it was going on, which would be expected since it interfered with his constitutional duty to verify the results of the election and he's a pretty strict constitutionalist.

https://www.blunt.senate.gov/news/videos/blunt-delivers-opening-remarks-at-rules-hearing-on-security-improvements-since-1/6-attack-on-capitol