My dounuts lasted 3 days the last time I swapped them out including new up-pipes and hardware and such.....
The solid up-pipe assembly is really the only true fix for the issue. At the very least get the up-pipes with the flex joints even if you still use the dounut/collector at the top.
Without anywhere for the expansion/contraction from the stupid heat cycle range you're dealing with here if you can get the dounuts to stop leaking (stop sliding) you will crumple the bottom of the up-pipe when it expands and has nowhere to go.
I will never once in my life "replace" the dounuts on one of these trucks again. Total waste of my time. If you do it twice you're well on your way to having bought a one piece assembly in stainless, and completely could have bought the international assembly with the flex.