IMO, the best coaches with a background in offense have more than just QB background. They have spent time coaching multiple position groups and have a much better feel for the team and not just the QB room.
Andy Reid was an OL coach, McVay was a WR and TE coach before becoming an OC, Kyle Shanahan also coached WR and then spent like 9 years as an OC before becoming a HC. LeFleur was mostly a QB coach but he at least had like 15 years of coaching experience before becoming a HC.
I get leery of the guys that only have a QB coach background and single digit number of years as a coach. That rarely works out well when it comes to running an entire football team and leading an organization. That was one of my concerns with Slowik.
Ben Johnson is one of those guys with a varied background in offense having coached TE, WR, and QB. If the Bears have a chance to get him, I think it would make a lot of sense.