I don't think they will show up on the 20th. This new iMac and MacBooks need a more important space, so I guess we will see some of them at WWDC or at the end of the year, when are the most important product presentations.
Back when all events were in-person, I would agree with you that MBP and iMac would wait till WWDC because they have done it plenty of times before. But now with these "pop-up" events, I see no reason why they could not show the new MBP and iMac alongside Air Tags and iPad Pros and whatever else they have planned.
I think they will try to keep the lineup as steady as possible, sticking to the old designs but with new innards. There is enough thats new even without a new chassis. A new iMac design might come 2022 or even later.
If that was the plan, they would have announced a 21.5" iMac with M1 alongside the Mac Mini, MacBook Pro 13" and MacBook Air last October.
No, M-powered iMacs will incorporate a new design alongside the M-powered 16" MacBook Pro and the new 14" MacBook Pro.