wifi network priority order changes when I reboot

Since this is a Home Wifi Setup - is there a reason the Guest Wifi is even listed in macOS or for that matter in iOS devices?

What would happen if on the iOS device the Guest Wifi was removed outright and on macOS you re-list the Family to Top and test by restarting the computer and seeing if it changes back to Guest at Top or remains as you have chosen Family Top and Guest below.

One can also add the Guest in iOS later

I will give you an answer! On your Mac go to System Presences->Network pan on to top of the panel and move the toggle bar to 'Cotton Name' naming it whatever you want, then save that because it will act better in network settings!

It appears you Want to see the Guest Network in Wifi Settings for some reason.

Personally, have Disabled the Guest aspect of the ISP Supplied Router. Reason.

Where I live in a Condo, my Wifi Sniffer Application can see 50 Other Networks in my area and 50% are Guest Wifi.

That means the area basically has 100 Wifi Networks all broadcasting and interfering signals with one another.

Here are limits and then there are unreasonable limits to Saturation of Wifi Signals.