Thanks! I ended up just running the makemkv container via Docker. It worked on my Raspberry Pis. The speed is great, but to get everything working properly, I have 1 Pi per drive. Even in TrueNAS, I was unsuccessful at consistently getting multiple drives connected without issues.
In the case of this Raspberry Pi, it simply didn't have enough power for all 3 drives since 2 of them were powered via USB (not external).
The hardest part is figuring out how to have the Docker container download new keys when my ISP doesn't allow it. I'm thinking, like my desktop, I can connect it to my phone as a mobile hotspot.