Any way to share only part of the screen on a call? IPad

To reiterate - screen sharing shares your entire screen.

Sharing your screen during a FaceTime call is certainly possible. Details of how to do so are described within the iPad User Guide:

Share your screen in a FaceTime call on iPad - Apple Support

If you have an iPad Pro that supports Stage Manager, you will have greater flexibility in arranging your App windows between the iPad and external monitor (if used).

Turn Stage Manager on or off on your iPad - Apple Support

Move, resize, and organize windows with Stage Manager on iPad - Apple Support

Did you review related information within the iPad User Guide - linked above? Screen sharing shares your entire screen.

Share your screen in a FaceTime call

1. During a call in the FaceTime app  on your iPad, tap the screen to show the FaceTime controls (if they aren’t visible), then tap .
2. To share your whole screen, tap Share My Screen.
3. A countdown from 3 to 1 appears on , then a small image of your screen appears in the FaceTime call. The others on the call can tap it to enlarge it and view your content.

To stop sharing your screen, tap 

You don't need to use an external monitor - but as stated, you may have greater flexibility when using one.