Time is largely going to depend on what muscle has been used to make it. Here (Australia) it's usually silverside (leg muscle) or brisket. Both of these are heavily worked and require a long cook for the collagen/gelatin conversion.
I usually soak my corned beef in fresh water for 4-6 hours to remove some salt and then cooked it sous vide at 60C/140F for 48 hours for brisket and up to 60 for silverside. Falling apart has not been a problem.
You'd do much better 'dialing in' a time that suits you by keeping the temperature stable and adjusting the time backwards rather then jumping around with two different variables hoping to find a pair that coincide with your desires.
So, if 140F x 40 hours was too fall apart for you, try 36 hours.... then 30... 6 hour blocks should be enough to have a discernible impact on the outcome.
And don't forget to keep notes on process and outcome. Oh, and make sure your carving knife is super sharp.