Hi tristan_cordeboeuf !
Elephant muscles sim looks interesting! when i was experimenting elephant skin behavior sim found that keeping the fat layer smooth and simulating wrinkles on dermal layer was more controllerable and manageable.
for the fine wrinkles, normal geometry was very dense. It was difficult to sim and iterate. So, I decimated zbrush high poly (which was in millions) to a required polycounts so sim time and its iterations can be manageable.
After decimation i had to spend time in cleaning up , Ziva tool "run mesh analysis " and maya "average vertics" was of great help.
I hope this might be helpful.