Establishing sustainable trade is a very necessary thing; for a nation, it is certainly good to produce and consume everything domestically, as it prevents dependency on others.
However, if one wants to gradually start building a self-sufficient country from scratch, then it is a waste of time and resources, which is not a good choice.
Producing surplus resources and using these resources to trade for other necessities needed by the country is what many nations choose to do; the Demon King Territory does this, the Silver Moon Tree Court does this, as do the Dwarves and Beastmen.
Except for the empire, there is no exception... This naturally does not mean the empire has all sorts of materials and grows all crops, but rather it simply does not necessitate trade.
Because civilians don't need "abundant food" to increase their happiness.
It's enough if they can eat to fullness (not starve).