I know nothing about the stand, but it sounds like you should get the 9th gen and be prepared to replace both the stand and the iPad in 2 years. That is to say: go with the iPad that will be supported longest. You’re handling people’s financial data on there so you shouldn’t be using a device that’s not getting security updates. You’re risking their data and potentially opening yourself up to liability if your device is breached.

I would go with the iPad 9 just because it has a more powerful chip (A13 Bionic) than the Air 3 (A12 Bionic). Performance wise they may not be too different, but you should be able to squeeze an extra year out of it.

My iPad Air 3rd gen screen started malfunctioning, so in January 2022, I bought the iPad 9th gen because I didn't want to part with my Logitech keyboard case.

The first thing I noticed was the screen's reflectiveness. While a matte screen protector helps, it doesn't completely replicate the antireflective screen of the Air 3. The difference is noticeable when the two iPads are side by side but it’s not a lot.

Another issue is that the screen protector isn't as smooth as glass, so you can hear it when swiping in a quiet room. However, since I mostly use my Logitech keyboard with a trackpad, I rarely touch the screen. It's not a major issue, but worth mentioning. There might be better screen protectors available.

Performance-wise, I did not notice any significant differences for my daily activities. However, my iPad has recently started slowing down and freezing when watching videos and using Safari after about 2.5 years. Backing up and restoring it has helped. Maybe you could try it, and see if it helps, if you haven’t already.

I chose the 9th gen primarily because it fits my existing case perfectly and has performed well for browsing and watching movies for over two years.

I would recommend the 9th gen as it will get longer OS updates compared to the iPad Air 3 and you can use your existing stand. But you might have to replace both in 2-3 years.

I have both of these and use both regularly.

I use the Air as a daily driver to take notes and do basic stuff (email, chat etc) at work. It’s just good enough but a bit sluggish.

The iPad is noticeably faster - unsurprising given it’s 2 years younger - but much heavier, hence I prefer the Air to take with me. As you’re mounting it on the square stand I’d go with that as you’ll get longer out of it.

I recently replaced my 9th gen iPad with a new M2 Air - and the jump in performance and snappiness was huge. The 9th gen is ok for the money but not really futureproof since it is one of those devices with only 4GB RAM. So when multitasking it could happen that iPad OS closes apps in the background. What this means can range from simply reloading a tab in Safari to losing partially entered data in other apps.