I decided that trade should only be updated when each round begins, so it isn't a benefit to take your turn late.
Actually everything might as well just be updated when each round begins... Would you prefer if your income was fixed at the beginning of the round, or should it be possible to change it during that round if someone who took their turn before you attacked you? And should trade be fixed or change during the round as other nations probably increase their production?
I would prefer it to be completely fixed, I think.