I own an iPad Pro 9.7 128 GB with cellular with a Smart Keyboard and it's a better device than the iPad 2018 for around ($384 with everything). While I got a used model with a slightly worn battery, it's still a very capable device, especially under iOS 12 as it gets around 5200 in multi core for Geekbench 4, which is slightly slower. Keep in mind the iPad Pro 9.7 does have a faster GPU than the iPad 2018. Of course, the iPad Pro 9.7 has better speakers, smart connector, laminated display with P3 color gamut with True Tone and the iPhone 6S camera. The 2018 has a worse display and I can't stand the air gap, especially since my iPhone 8 has a laminated display.
The iPad Air 3 is just a bigger version of the iPad Pro 9.7 in a way, except that the camera is downgraded to 8 megapixels and no quad speakers. It still has true tone, P3 color gamut and smart connector.