Is it possible to designate a Wi-FI SSID as metered "mobile network"?
This option actually exists and I have just tested it on Android 10 (AOSP). The trick is to set a custom DHCP option 43 with the text value "ANDROID_METERED". What I find a bit confusion is that Android does not show you that it is on a metered Wi-Fi. I verified it with the NetworkMonitor app.

Someone documented this further: https://www.lorier.net/docs/android-metered.html