@Tiny-0 default defines a function in this case.
"A selection made usually automatically or without active consideration due to lack of a viable alternative.."
If you have no alternative, ie a different/better route that says go here.. Then default route is what is used.. That is a function ;)
Sorry that the way to set policy routing is too much work for you.. But doing something with like a source network route in the routing table I do not believe works for pfsense, because I don't believe it uses multiple routing tables..
If setting the option to use a specific gateway is too many clicks for you.. Create a rule, and then copy the rule and just edit the other aspects of the rule.. The copy will maintain the policy route setting.