I currently own both 2020 Air 4 and 2021 Pro 11 (use them alternately when the other one is charging). Apart from extra RAM and 1TB storage, I don't really care about any of the 2021 Pro features (and I've had access to quad speakers since 2016 and ProMotion since 2017). The 0.1" display difference is negligible. Brightness is a moot point considering I tend to keep it between 0-25% (even have Reduce White Point enabled at night).
At one point in time, I had an Apple certified refurb 2018 12.9 1TB and Amazon open-box 2020 12.9 1TB. That was within return period while trying to decide which to keep between 2018, 2020 and 2021 12.9. The refurb 2018 had noticeably worse battery life compared to the 2020 12.9, 2021 12.9 and 2020 Air 4 with the same settings/tasks. Iirc, the 2021 Pro 12.9 and 2020 Air 4 typically gave me 15+ hours onscreen time and standby battery drain was around half of the 2018 Pro 12.9.
Previously, I also had the 2017 Pro 10.5 (acquired early 2019) and 2019 Air 3. Battery is shot on the Pro 10.5 (A10X/4GB) while Air 3 (A12/3GB) still has very good battery life.
If I could BTO an iPad, it would be a mashup of the Air 4/mini 6 and Pro 11:
- Apple A15
- 16GB RAM (8GB is acceptable)
- 1TB storage
- TouchID power button and FaceID
- same camera setup as mini 6
- stereo speakers are fine
- Price: $1200 or less