Is it possible to designate a Wi-FI SSID as metered "mobile network"? I am wondering if there is a method by which I can designate a WLAN/WiFi connection as "mobile network" based on its SSID for the sake data usage policies by apps or the Android system in general.

Typically when you have a data plan with your mobile provider, your phone's LTE (or  3G ...) modem will connect to the provider to enable you to have internet access.

Now if I am using a mobile LTE/WLAN router instead, my phone assumes that it's on an unmetered WiFi uplink and apps/the system will (mis)behave accordingly.

Is there a way to tell the system that when it connects to a particular SSID it should treat that as "mobile network" in regards to how apps may use data?

Phone: Samsung S9+ (SM-G965F), Android 9

NB: the settings from this answer don't exist on my phone.
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