Extensive cattle ranching is the number one culprit of deforestation in virtually every Amazon country, and it accounts for 80% of current deforestation

https://wwf.panda.org/discover/knowledge_hub/where_we_work/amazon/amazon_threats/unsustainable_cattle_ranching/

Plant-based food production uses less cropland

The research suggests that it’s possible to feed everyone in the world a nutritious diet on existing croplands, but only if we saw a widespread shift towards plant-based diets.

[...]

If everyone shifted to a plant-based diet we would reduce global land use for agriculture by 75%. This large reduction of agricultural land use would be possible thanks to a reduction in land used for grazing and a smaller need for land to grow crops.

https://ourworldindata.org/land-use-diets

Complete proteins matter doesn't really matter all that much in practice. Things like soy are complete on their own, and things that are technically incomplete proteins like beans can be made complete with rather little like even rice. You don't need to be getting every amino acid in with every meal. If you eat the amino acids at some point in the day, you will be fine