icloud backup fails with 80% available capacity

Your post is very confusing. Here is why.

• “I just want our iphones & ipad to synch so we see our email & messages on all devices.”
  • Seeing email and messages on all your devices has nothing to do with iCloud backup. It requires housing this data in iCloud. Backups are used to restore data in the event of a catastrophe or replacing a device.
  • The use of “our” in your sentence is alarming. An Apple Account should be used by one person. Each person, regardless of age, requires their own Apple Account. Make sure that each family member has a unique Apple Account - Apple Support
• “I backup our iphones & ipad on my imac”
  • It’s OK, but uncommon, to back up your devices both on a computer and iCloud. I guess that’s a way to be extra careful.
  • The use of “our” raises the red flag noted above.