Connect my watch with family iPad for Fitness+ Ive got Apple Fitness* shared with my family.
My wife has an iPad (with her own account, but part of the family sharing group) running 14.3 - capable of running Fitness+.
Can I pair my watch to the iPad for the purpose of workouts only?
No.  At this time, you can only pair the Apple Watch (all series) to an iPhone.  The Watch is deemed as an iPhone accessory by Apple.
It's not even possible to install the Apple Watch app on iPads.