I want to use my own reader but keep the teleprompter on

Easy, you can absolutely do both.

While you can't disable voices entirely, you can turn their volume down to zero. The scene will still play normally and the teleprompter will scroll when you speak.

If your reader finishes before Linus expects your cue:

1. Go to Run Lines.
2. Tap the slow line → three dots ⋯ → Adjust Speed → move the slider right to make it faster.
3. Tap Apply All to change every line, or tap once on the script to apply just that line.