There's another Android 13" eink tablet I know of, the Bigme X6 but the Onyx Boox Max Lumi or Max Lumi 2 are both cheaper and should basically do the same thing (load any Android apps). The Fujitsu Quaderno A4 is an option too but it only supports loading PDFs and has no third party apps (w/o rooting it).

The Dasung Paperlike 3 HD or Onyx Boox Mira are 13.3" monitor solutions as well, something to consider if you want to hook them up just as output to run w/ arbitrary software (both companies have 25" versions as well).

For 13" e-paper document readers, there's the Fujitsu Quaderno A4 or its predecessors, the Sony DPT-RP1 / DPT-S1. They are PDF only, since they were intended for professional use.