Yeah, I had a 2018 12.9 that had a weird battery module failure, and I decided to just go ahead and pull the trigger on a 2021 12.9. I had no complaints of speed, only the 4GB of RAM and closing apps. That and the display for movies are the only differences I notice between 2018 and 2021.The A12Z that powers these 2018 Pros is still really fast. I have two of them and have never felt the need to upgrade to M1 or M2. These are probably a better choice than than the new base and Air iPads for many.
ProMotion and Gen 2 Apple Pencil support beats out these newer devices in my opinion.