My guess is that that BOTH the XR/11 will be discontinued fall 2021; with a 6.1 inch SE plus taking a price point around where the XR is now, and the 12 being cut down in price and sitting just above where the 11 sat. There's no hard and fast rule which says that Apple must keep the 11 for 3 years till 2022 just because they will have kept the XR around for 3 years. I'm going to say that the SE plus will come with a dual camera set up also.
I really can't see the SE plus being based off the 8+ either; I think it'll be a 6.1 inch 11/XR style design. Could be wrong.
My other guess, is that they'll only discontinue the XR, keep the 11 around till 2022, then bring out the SE plus then.