Problem
I think its a bit stupid that you have to buy influence in a random port that has nothing to with your nations interests. 5mil gold is a significant investiture in a port, so why can it be in a port my nation/I want.

Solution
Nation or King picks ports your buy influence in
or
The mission is not port specific

how about focus the mission on most populated port of the nation at the moment.

Or the port that has the most influence in your nation.

Or the port of your nation with the LOWEST influence.

There is something inaccurate in this suggestion; The port selection is never random. There are already modifiers that affect the next port selection. (like most/least influence etc)

We can however make it up to the council decision, no problem.

I see no issue why the Nation Council shouldn't be able to determine the ports where nation mission influence is applied. If no selections are made, the default methodology is used.

Approved.

Approved - Vane

Approved - Clock

+1 Approved Skyhawk