How to remove a lost iPad from trusted devices

Use your other trusted device (like an iPhone or Mac) to review your account. Go to Settings, then tap [Your Name] or System Settings, tap [Your Name], then tap Sign-In & Security to review your security settings. You can remove a trusted device from the list of devices shown there.

If you had been using Find My,

• Marking your iPad as Lost right away would have been a good idea. According to Apple, "When you mark your device as lost, your device is put in Lost Mode, which means your device is locked with your passcode, and payment cards and passes that you use with Apple Pay are suspended."
• Removing your iPad from the list of devices associated with your Apple Account would not have been a great idea. If you removed a iPad that had Find My turned on from the list of devices associated with your Apple Account, that would have immediately cleared Activation Lock, letting any thief or dishonest finder reset the iPad and "make the iPad their own." Once you remove an iPad from your list of devices, there is no way of remotely undoing this action.

How to find your lost iPhone or iPad - Apple Support

However, under the circumstances, it sounds like removing the lost iPad from your account is the best that you can do.