Unfortunately, having membership in two separate Family Sharing plans doesn't seem to fix most of the problems you're reporting here. Kids can still share Wi-Fi credentials and passwords, and approvals are still going to be specific within a Family Sharing plan.
You can secure your Wi-Fi somewhat better by loading and using a profile on the kids' devices, if you have a Mac to create that. Other available profiles and settings can restrict other capabilities of the devices, though not all.
Create and edit configuration profiles in Apple Configurator - Apple Support
Profiles are akin to parental controls, but are intended for businesses and organizations.
iOS 18 does provide screen sharing for remote help and support:
Share screens using Messages on iPhone - Apple Support
Request or give remote control in a FaceTime call on iPhone - Apple Support
As for blocking locations and some of the rest, maybe discuss that with the attorney that handled the divorce?