FWIW:

The A750 does share the same gear train as the A750. So it does have the 6th gear internally. The problem is that it is built missing one internal part (not a gear, but a clutch ring component), and most importantly the A750 has a different valve body. So unfortunately it isn't just software. It doesn't have enough valves in the valve body to shift into all 6 gears. I don't know if it would shift through all 6 gears if you just swapped valve bodies. The housing is exactly the same, so they would pretty easily bolt in place of each other. I don't know why they stick with 5 gears in the 4R when they put the 6 speed version in other foreign models. I think they really just DGAF about the USA market. I can't really think of any other reason not to use the 6 speed.

The Tundra 5.7 is the AB60 rather than the A760. It's a bigger heavier transmission. The A750 was used behind a warrantied supercharged 4.7 v8 in the past gen 4Runner. And the A760 is IIRC what was installed behind the 4.6L tundra. I think the new 10 speed will probably be used in the full range of toyota BOF going forward though. I'd guess it'll be cut down to a single transmission. But the 10AT is fantastic, so I'd be really happy to see it come in the next gen 4Runner and Taco.

The AB60 in the tundra vs A750 planetary gear set below is a good visual of the comparison. Both are nearly identical in design. The AB60 is just scaled up a size category. Both are very reliable. Probably the most reliable production automatic light duty truck transmissions ever built. I can't think of any that would be better.




Interestingly - I've read that the 9 solenoid valve body on the AB60 and A760 suggest that it would only take minor tweaks to make them shift through a potential of at least 8 gears. Typically a 3 planetary gear transmission would have at least 8 possible combinations as long as you can choose every clutch pack combination. So it's at least plausible that even the A760 and AB60 have the potential for more gears somehow. I just don't know enough about them to really break down if that's possible or not without spending a time looking through the schematics.