We're currently in the process of terraforming earth to make it slightly less habitable.

Terraforming means making it Earth-like. Did you mean terra-deforming?

As far as I'm aware, nobody's succeeded in turning deserts into farm land on a large scale. Various parties keep trying but the water, fertilizer, and maintenance costs are astronomical.

Israel has turned much of it's desert into farmland and forests. For example, the Yatir forest is one: https://en.wikipedia.org/wiki/Yatir_Forest

All of Los Angeles/most of southern California was a desert, wasn't it? Massive water redistribution is the reason tens of millions of people can live there. Much of California's agriculture comes from the same water redistribution projects.

Plus, there's no life that we know of on Venus. A bacterial population capable of living, evolving, and growing exponentially in that environment would be a huge change.

We definitely have. There's huge environmental impact, but that is to be expected.

California's Central Valley project is an example, and China has some that are even larger.