Is there any advantage to their fixed payment plans at all vs just using this model via 3rd party providers via openrouter, given how relatively cheap they tend to be on a per-token basis?

Providers like DeepInfra are already giving access to 5.1 https://deepinfra.com/zai-org/GLM-5.1

$1.40 in $4.40 out $0.26 cached

/ 1M tokens

That's more expensive than other models, but not terrible, and will go down over time, and is far far cheaper than Opus or Sonnet or GPT.

I haven't had any bad luck with DeepInfra in particular with quantization or rate limiting. But I've only heard bad things about people who used z.ai directly.