Better fuel economy with the 7-speed. If you're building the car for a bunch of power the gearbox can take more torque (limitations besides just clutchpacks). Some 7-speed cars come with a dsg "catch can," some don't.
You can retrofit the cluster. The injectors that come in the 19+ cars suck balls, but you can swap to pre-facelift ones. The primary O2 sensors are different (not swappable) and can be hard to find at times (very minor issue, but still annoying). I believe some other harnesses are slightly different or revised, but nothing else I can think of where you can't use pre-facelift parts.
The o2 sensor thing is due to the 2019 cars using a newer ECU but at this point there's not much difference when it comes to tuning/feature parity.
I haven't been able to find any other significant differences with the DLRA (2019), DJJA (2018) and CYFB (2015-2017). Very early (2015) IS38s might have had more issues, but they're all a crapshoot.