If ProMotion isn’t important to you, then see whether the new Air 5’s features will entice you. I know people will say it’s a ’downgrade’, but the A15 chip will be much faster than the A12X. The camera front and back will likely be superior on the Air 5.
Alternatively, consider buying into a M1 Pro - or - wait to see what spec the rumoured 2022 Pro will have.
I wouldn’t recommend an iPad 9, the RAM capacity is definitely a huge step-back from the 2018 Pro. Also the lightning port is a huge regression.