Apple has far surpassed the diminishing point of returns on iPad hardware. 2018 was the peak. To get those 2 years to upgrade in year 3 it is going to need some major software improvements, especially in light of the M1 Macs.
The 2018 pro's already have True Tone, 120Hz, A12X (Fast enough for every iPad app on the App Store and more), 4GB RAM (Again, enough for every iPad app in the store to operate normally), Apple Pencil support, keyboard/trackpad, USB-C. More than enough for any "pro" iPad user, and way over the top for any normal consumer.
To be honest, if apple slightly changes the dimensions of the 2021 pro I won't buy it, because their ridiculous $300 keyboard then becomes device specific again, and they are just trying to grab more $$$ on accessories.