You can trade everything for diamonds. Harvesting with boosters, including buildings, is great. Otherwise, the amount of goods you can trade counts. You harvest trees and vegetables. In what quantities? If you have a fully developed farm, this should work well. However... what's the fastest way? It still requires diligence and patience.
There's no single right way anyway, because everyone farms differently. With vegetables, you can opt for the shortest harvest time and then farm nonstop. That wouldn't put me in the mood. Harvesting lots of milk and wool and boosting animals makes quite a bit of money for me. Same goes for flowers. I trade directly in the shop, not through the buildings on the farm.
I also need a few more slides for the last 13 plots of land on my fourth farm. They're very expensive. But if I farm all the farms, I'll get a good amount. It'll just take a while. But I'm not in a hurry. I don't want to rush through the game. Harvesting milk and wool with boosters are my favorites.
I forgot to exchange gold for diamonds

Make the investment in mines. Harvest with boost.

I would advise harvesting a variety of things on a short timer to be able to trade at town shops to maximize diamond acquisition since a single shop will run out of diamonds. Also harvesting the mine and keeping up with House tasks will increase the diamonds to time ratio.

Rabbits are very effective. Waiting time until wool harvest: 45 minutes for the fastest species. The shop for selling wool brings in a maximum of 600 diamonds for 100 wool - if you harvest the rabbits with a boost, this is a very quick way to get diamonds.

Fully unlocked mine gives about 20K every two hours.