Unfortunately I also have a bad experience with the Sapphire RX480 8 GB Nitro models. Out of 24 of those cards, 4 pieces died for me. I maintained them regularly, blew them with a compressor, put the best thermal paste "Thermal grizzly kryonaut", I have a very high quality and reliable power supply. I am very disappointed. So I went back to MSI again. Now I only use the MSI 5700XT Gaming-X series graphics cards . I used to have several MSI 7950 Twin Frozr IIIs and have been mining with them for years. None of them ever died. Fans did go, but graphics cards never did.
How did they die on you exactly? Did something blow up OR did you just get a BSOD on bootup?
If you got the BSOD or they started to crash on cold start... I think I know what happened.
Most likely since you said you replaced the thermal paste, you might of pulled a little too hard when seperating from heatsink and created micro-cracks. At first you don't get issue because they are small. However on every heat cycle, the cracks get larger and larger. Eventually one day when you stop mining to do a reboot, it crashes all of a sudden or fails to boot or you get BSOD. Then you find out the GPU is dead.
I also killed a few of my GPUs this way. Now i never do the thermal paste job unless the GPU is less than a year old.
I didn't change the paste then. One piece died in operation and 3 pieces died after blowing with a compressor (I blew them regularly until then, every 3 months). All pieces died after the 3-year warranty expired and not all died at once. For example, one would die today, the other after a month - 2 days and so on. They do not give any video signal, the fans do not turn. It's not a GPU failure, that's what the master told me. He fixed all the graphics cards and says there were several breakdowns on each.