the nobles (the un-developed class in this game) set an additional bonus for x imported specific cargo. this extra will come out of the treasury.
when port growth is linked to imported/consumed goods, than ports need a steady import. growth means a larger income to the nation, so a fine balance between bonus and port income has to be found.
next, as suggested before, port population defines port production. a decline in population will cause a reduction in produced goods.
population still can be regulated by TI, BD, immigration and goldcoin investment. but a failure in supplying the population will lead to scaled population decrease. (1-5% failure, 1 % reduction;.. .. ; 50-60% failure 60% reduction) the opposite (more goods imported than needed) will lead to a slow increase of that port. Ports will balance out on a level that they can sustain. the more merchant they can entice to supply them the better the port will do.
large nations will find it hard to supply all their ports by themselves, it may lead to more nations on the map, or to schemes of wealth sharing

party cards should not count as supplying the port for it is consumed in one extravagant party.
this overall scheme will lead to a new balance that depends and grows on the amount of trade fleets. the more fleets are trading the larger the ports can be.
the bonus the nobles can add to stimulate import is a way to lure merchants to favor their port above other ports.
the competition in this scheme is not between merchants directly but between ports. they have to lure merchants to supply them to maintain current size and continued growth.
this competition could imo stimulate the development of ports and blockades as well. ports have with this scheme reason to blockade other ports. however, ports and blockade is a different topic that is unrelated to this topic.