I own a 2019 3500 HO/Aisin with factory air bags. I had CJC Offroad install the "
Carli Suspension 3" Pintop 2.5 Suspension System 2013-2021 Ram 3500 6.7L Cummins with Factory Air Assist." I am running 37" Wildpeak AT3W tires on Method 701 HD rims.
Based on your requirements, I would get a 2500 due to the much, much more comfortable rear suspension, tunability of the engine, and the comfort/tunability of the 68RFE.
The rear leaf springs on the 3500 are super stiff when not towing, which is to be expected. Even after getting the Carli suspension installed (with better leaf springs), I think a 2500 with a Carli system is much more comfortable.
There are no rear-end gear sets available for the 3500 at this time, so you will be stuck with the 3.73 gearing on 37" tires. I found that this isn't a great combination with the 37" tires. I would like to install a high-4 or low-5 gear ratio, but nobody sells gears for the 12" AAM rear differential, yet (as far as I know).
There are more tuning options for the standard output diesel than the high output diesel.
The Aisin transmission is great when towing my 15,000lb 5th wheel, but when not towing I really wish I had the 68RFE. I recommend you watch
Eddie's Life YouTube video comparing the Aisin to the 68RFE. As you said, there are no tuning options for the Aisin, but there are lots of tunes for the 68RFE. Also, a built 68RFE transmission+converter is a lot cheaper than a built Aisin transmission+converter. The oil change interval on the Aisin is 30,000 miles which is double the service interval of the 68RFE.