Syncing some but not all my devices together

As described, you would be ill advised to consider using your existing AppleID for your proposed dedicated home-automation setup. Using a secondary AppleID for the new devices allows you to maintain separation, this keeping your personal and sensitive personal information away from devices that will presumably be accessible by anyone.

Should it be necessary to do so, you can use Family Sharing to selectively share access to just purchased Apps and Apple services.

Devices that share the same AppleID share the same security context - and as such both your personal/sensitive data and files will be accessible - along with the ability to access or compromise the AppleID account itself - from all devices that are using the AppleID.

Reliance upon Guided Access to inhibit access to data and AppleID account is unsafe. If you can successfully break-out of Guided Access (this potentially being a trivial exercise), everything is accessible.