I went from a 2008 Toyota Tundra DC 5.7L TRD to a 2020 GMC sierra 1500 SLT with the 3.0L diesel. Night and day difference. I too was waiting for the newer Tundra but after 12 years, I just didn't want to wait any longer. I also had some issues with the Tundra I won't get into here. Onto the differences!
Ride quality and driveability: The ride quality of the 1500 is much smoother and feels more planted than the tundra did. Also the 1500 feels much more nimble in handling. Feels like driving a smaller vehicle. The engine and transmission are much more sensitive, downshifting is quick when needed. But also because of the torque, there are many times I thought the truck would downshift to go up a hill, but never had to. Quieter ride as well.
Fuel mileage: This is one thing that really stood out, as you probably have seen from the other posts. I average 21 MPG city and over 28 MPG hwy with aftermarket wheels/tires with a truck topper, so around 24 MPG combined. My tundra averaged 13 MPG combined with just aftermarket tires (LT). On a few long road trips, I have gotten over 30 MPG.
Engine: I really like the 3.0 diesel from GM. I test drove the ram diesel as well and found the turbolag and fuel economy restriction annoying. From a dead stop, turning onto a hwy or faster side road, there was a good approx 1-2 sec lag from when I would hit the gas peddle and when the power would kick in. On my test drive, that lag almost made me miss my window, the on coming traffic had to slow down for me. I promptly drove back to the Ram dealership and returned the truck. The lag from the GM diesel is much less restricted and feel like it gives you some power to start with when you hit the peddle at first, then really gives you power as the turbo kicks in.
Infotainment/controls: The infotainment/controls aren't as fancy as some of the other trucks (Ram/Ford), but I think the transition from the Tundra to the 1500 is a nice bump in convivence. Still touch screen and very responsive at that. But you still get the tactile knobs and buttons for most the stuff you don't need touch screen on. Compared to the almost all touch screen of the 12" display ram, which I hated and actually think that it's a danger to driver attention. For me the 1500 was the easiest transition in terms of buttons/knobs and touchscreen infotainment. You will notice the steering wheel controls are in different positions from the Japanese vehicles.
Offroad/hunting: Although I have the SLT and not the AT4, which should more offroad capable, using the truck to get to hunting camp and driving offroad is much more enjoyable. Hill descent control, auto 4WD, backup camera (AT4 I think has 360, would is even better), front and rear sensors, all help getting around in the backwoods without damaging the vehicle.
Truck bed: One thing that sometimes gets overlook is the truck bed utility. Inside the truck bed on the 1500 has multiple (12 total) tiedown hooks and additional areas to add more hooks. Also has bed lighting and plug available and pop up handle to get in/out of the truck. These are great additions and I've found myself using the multiple tie down locations alot.
THE BEST THING EVER: Multi-pro tailgate. I would say this one thing is the coolest item on the truck. So convenient and I use it almost all the time. Not just for getting in and out, but also for carrying lumber/wood products where I would have had to have a regular tailgate all the way down, I can use the multipro with the top portion down and quickly secure it and be on my way. Also it will save your back. I've loaded 50 lb bags of sand/gravel and pavers and it was a huge difference to be able to stand closer to the truck bed.
Towing: I have not towed with the 1500 yet.
I don't think you will be disappointed in making the switch. Goodluck!