Astro Pneumatic was nice enough to send me replacement mandrels even though they consider them consumables. I went ahead and ordered a couple of extras since I knew I had hundreds more rivnuts to install. I managed to make it through virtually every remaining rivnut on the chassis. There are only a handful left that I can’t reach with the Astro tool.
I broke one of the replacement mandrels by over-torquing the tool slightly. After that, I only used the tool with a 1/4″ drive socket wrench instead of a cordless drill. By doing that, I could pretty easily stop before the torque got too high. After that, I installed 200+ rivnuts without breaking a mandrel (more than the previous two mandrels combined).