I have noticed something about the comparison between Trade Port and Treasury - point that hasn't come up yet but I think is very significant. While a single Trade Port is almost strictly better than a single Treasury, the latter is easily spammable with little downside. However, each additional Trade Port you gain before Winter is worse than the one you gained before because it will be effectual for fewer turns overall.
As the most recent game Asper, LastFootnote and I played clearly showed, spamming Trade Ports is extremely slow and expensive and in Winter you lose a bunch of payload. So not only am I convinced now that Trade Port is balanced as-is, I'm also growing fond of the idea of giving you an incentive to buy them in multiples.
Each subsequent Trade Port is effectual for fewer turns... but that applies to
every card you buy. The closer you get to game end, the fewer chanecs you have to play the card, the fewer times it'll have to make a difference. That's already a guiding principle for end game purchases. If the game will end before you see your next buy, maybe you should buy a Duchy instead of another Treasury.
I don't think it's as big a deal as you are making it out to be though. That is to say, I am not convinced that Treasury (or Market) are significantly more spammable than Trade Port. Yes, each subsequent Trade Port is less effective than the previous one because there are fewer turns for it to be active until Winter... but that's not the comparison that matters here. How many turns will it be effective compared to Market or Treasury? Trade Port is gone for Winter, but Treasury is gone for a full shuffle after you buy a Victory card. Market is gone for a full shuffle after
each play. So it will depend on how long your shuffles take. And note, if the game ends before Winter, Market and Treasury will have been almost strictly inferior.
That's why I asked my earlier question about how much Winter affects the strategy. Winter is turns 16-20, but I think the kind of board where Trade Port, Treasury and Market are best is also the kind of board that is likely to end early, where you get maybe one round of Winter and then it's over. That's why I was suggesting having the card advance the seasons more quickly, or have a weaker/negative effect in Fall.
All that said, I do get that it still isn't as spammable in that there
is some opportunity cost with that first play of each card. It's more dangerous to buy a bunch before getting them into play because they don't do anything on the very first turn you play them, and if you're lacking +actions then it might end up being a dead card if multiples collide due to overzealous buying. I'm just not sure that balances out all the positives. The similarity to Treasury (and Market - also a good comparison) is a concern, especially since Trade Port is arguably so much better. It's difficult to judge because its unique drawback is very different from the cards we are used to.