Even though I wasn't particularly thrilled by the 9th iPad, I concluded that it is the right model for us at the moment; the air 4 is expensive while not offering crucially more (for me), the air 4 is in fact just more luxurious; and since all iPads (pro/air/base) run the same iPadOS there is not much to gain in functionality. The air4 would also mean I need another usb cable to use with my digital piano, which I would have to switch if I would like to connect my iPhone 11. I actually prefer the lightning connector at this moment.
I noticed last year that the price of the iPad 8 didn't drop (a lot) during the year, so I decided to buy this one as soon as it became available. For me the main reason was the ability to use the apple pencil.
I never ran out of the 64 GB of space on the air2, so the iPad 9's base storage option was exactly right for me.
First impressions: the screen reflects more; but as the air2 already is >5 years old, it's screen brightness became uneven. The new one has much better uniformity. In fact, the air2 wasn't perfect there from the start. So, if you would ask me at this moment, I would definitly take the iPad 9's screen over my old air2's. I don't notice the gap (to my surprise).
I don't have a cover yet. When comparing the air2 with it's thin back cover it feels the same as the iPad9 without. I know, this is not an honest comparison - but the feeling isn't day and night at least (and the screen size of the iPad9 is larger).
It is the first time I had to move data over from the old to the new ipad, and this process was quite smooth.
A nice side-effect is that I now also have a proper 20W charger for the iPhone.
The pencil works great, although I know and have experienced that the pro iPads have a much better feel when writing. Here you do notice the display gap, but to be honest it's not a real dealbreaker.
For the rest, the iPad is in fact just identical in use - just a lot faster than the air 2.
If you do not have a use for the pencil, and are ok with the speed of the air2 I would definitely wait until the next version of the iPads. Apart from the pencil there wasn't anything that I couldn't do on the air2.
I still am wondering if apple will ever make more distinction in terms of iPadOS functions between the pro and base lineup. If, for instance, there were a lot of apps that didn't run on the base model because of advanced functionality, my decision might have been different. But the music apps that I use will run more than fine on the base ipad, in fact, the headphone port is an advantage.
But I don't think this iPad will run for as long as the air2 did. It will depend on the direction Apple takes with the iPad and Mac roadmap. At this moment this is still very unclear to me, I already expected much more changes in iPadOS to make it more usable as laptop replacement, that didn't happen so far.