Computed value based off of current document (like conditional fields)

[Snugug]

I'm working on a field where I want to compute the value of a read-only field based on the input of another field (in this case, video ID based on input video URL). Ideally, I'd be able to set a value property, or something like it, with the same function signature as conditional fields. I've seen sanity-plugin-conditional-fields but that appears to rely on a GROQ query and this particular field is quite modular so I won't know in any given instance what the exact query path would be to grab the correct value, but knowing its parent is totally do-able.

Is there a plugin that will let me do this? Or something I'm missing in Sanity core?

Thanks!