In theory the UI->Settings->System->Network and under it “Network Adapter” is suppose to allow one to select the “default” interface/NIC. However it has been reported in the past that this doesn’t work (don’t know if it is still the case or not).
However, here is a Community Forum thread on this topic and I’ve selected a particular topic number that may help out. This particular user had multiple interfaces/NICs, each with a default route, but he demonstrates how nmcli could be used to configure a least cost route metric for the default route associated with a chosen interface/NIC so that it will act as the ultimate default. Note: this user didn’t actually do it this exact way, but you’ll get the idea.