Wow, that is some comparison. Impressive. Beware of rationalization. I detect a bias towards the Air 3.
Yeah, I sense it too that I have some subconscious bias towards Air 3. But even knowing that I have two weeks time to return it and it is not a problem, I still seriously consider keeping it.
I am not sure I would put much weight on some of these factors (like your in depth case review, there will always be more cases to chose from).
I never learned to like typing onscreen keyboard, so I definitely would like to get real keyboard this time but I definitely also need to be able to use it as tablet easily. So I'm not sure there will be much good choices for Air 4 and Pro's since the smart connector is on the back side. In Air 3 smart connector is on the side so keyboards attach magnetically to it but in Air 4 and Pro's they would have to make some complicated cabling and connector system in the case, so i don't think there will be much those, especially from known manufacturers like Logitech which makes very good keyboards BTW.
Apple MK is the only choice where it would be easy to remove iPad from the case but it offers no additional protection to iPad itself, it would require another case and I don't think it would be very convenient to keep switching between the cases. So considering all that Logitech Combo Touch for Air 3 with removable magnetically attached keyboard makes a lot sense to me.
But then, I have my own rationalizations, I really hesitate to buy last year's tech. And the trump card I rationalize with is... resale. You have indicated you get bored with tech in 2 years, and you downplay the importance of the A12 to the A14 to you (but hey, what new Apps wlll come out with AR and the A14?), but what about the person you sell/give this to? It will be easier to resale the Air 4.
I considered these too and I'm also a bit of a hardware addict myself wanting to get the latest tech but then again honestly the tasks I'm using my iPad for aren't really something that needs much power. I will not use AR and regarding resale value there is like 200 price difference today, after couple of years probably it is not more, could be even a bit less.
Also considering longevity of these machines I realized that iPad 8th gen introduced yesterday is too A12, so based on that I'd say A12 it going to be supported for 4 or even 5 next IOS versions, so probably Air 3 is the same. And after that machine is practically useless anyway if considering battery life and such.
I currently have iPad 6th gen (A10) and I never felt it is slow or anything like that. The worst thing with it is just lousy battery life and Touch ID that rarely works as it should.
Just saying, in case you need a reason to go the Air 4.
I would counter with, if it is a wash, or even close to it (might be in favor of the Air 3), then why NOT go with the better tech if it is essentially costing you nothing or very little. Just saying.
I'm also considering this so that between Air 4 and Pro there is also very little difference in cost too, especially when comparing 256GB models. But again I already considered before Air 4 was introduced that Pro is overkill for my needs and probably I would never use many of its features and also I hated the keyboard case issues I mentioned with non removable keyboards in those versions as one big thing.
In the end, price is not really an issue for me or deciding factor, I'm more like trying to justify myself what is enough for my needs even it is not the latest tech. For example I bought MBA last year and later on returned it and got MBP 2019 that was just released but later on I realized that fact that all I have done with the machine is practically used it for surfing on the internet and some other basic daily stuff, never really used much of the Pro features, Air I had would have been good enough. So I though maybe I should use some better consideration with new iPad I'm selecting for myself.