I might have a fix if you want to try it.

A few weeks ago it was working just fine. Now, when I open Daydream and cast to TV I get a black screen with glitched lines running up and down

First, was daydream casting working after Android 10 update?

Did you just recently install the new update for the Daydream APK (if you don't know, then check to see if you have automatic updates turned on, if so, then you recently updated the Daydream apk).

This is the version prior to the most recent Daydream apk (v. 1.20.190204006): https://www.apkmirror.com/apk/google-inc/daydream-daydream/daydream-daydream-1-20-190204006-release/

Installing it may fix the casting issue if it was cussed by the new version of the app.

You will have to turn off automatic updates (else soon as you install the apk it will auto update it) then uninstall the newer version of the Daydream app, and install last version from 2/19.

Let me know if it works for you.

(I had to do something similar to fix Google Play Movies VR).