Updated to CyanogenMod 12.1, tablet now stuck in bootloop with TWRP
I got the answer.

I installed Odin, then flashed the correct TWRP archive, from Download mode. When it restarted, it booted into CyanogenMod without any issues. I will only need to install GApps again because I did a factory reset, but that isn't an issue (I hope). I can only assume that the update messed up TWRP, causing it to loop.