I have a 2020 Custom Trail Boss with the 5.3L AFM and 6 speed transmission. If you want better gas mileage, that combo is not for you. I get on average between 12.8 and 13.3 MPG. At the time of purchase in my area, the 5.3 and 6.2 in either 8 or 10 speed transmissions were very hard to come by and the wait for them were a couple of months out. All in all I do not have any regrets about the purchase, just small gripes like in cab storage and the lack of cup holders in the back.
Based on what you have already researched and listed out, I’d say go for the LT, that would be the better option for you. You’ll get the gas mileage you want the either the 8 or 10 speed transmissions and you will also get a little bit more features and creature comforts than what the Custom has.