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Gift taxes between partners are very low in my country. Like 1% or even exempt from taxes under a certain amount. So that is safe.

With regards to tax residency.. In theory she is now a tax resident in my country (EU) because she lives here and works here. She came here 6 years ago and was a student. In all those years she was a tax resident in Brazil, even though she did not work there anymore. But in 2023 she became employed here in Europe. She automatically became a tax resident here and her taxes are automatically deducted from her monthly salary here.

But in the mean time, she has not signed an official declaration of no longer being a tax resident in her homecountry. I recently read that citizens who are no longer tax residents there can either sign a permanent exit declaration, or a temporary one. We don't know which one applies, because we might move to Brazil in 5 years.
We are not very worried about double taxation because there is a treaty against double taxation between our countries, and we can prove that she is already taxed here.

But if she connects her Binance.com account to her Brazilian bank card, I don't see how Binance would inform European tax agencies. Because Binance only knows her nationality, not her tax residency. And technically she has also not given up tax residency in her homecountry. If Brazilian authorities demand taxes on her crypto transactions, we can prove to them that she lives in Europe and pays her taxes here. The treaty applies and therefore they can not tax her. Meanwhile we should report her crypto profits on Binance.com to European tax authorities, but I don't see how they would find out.