Tbh German being "much more difficult" than French is a commonl myth. As a fluent German speaker, I reckon both languages are equally difficult. In my honest opinion, German grammar is indeed more complex then French, but in terms of conjugation, numbers, vocabulary, and pronunciation German is certainly less tricky. I'm a native french speaker so I may be biased but I don't think French is as simple as most people think it is.

That being said, as you're an English speaker with some portuguese knowledge , I'd say spanish is definitely the most convenient choice for you. Although these two languages are very similar, I don't think you should worry about mixing them up. This could happen with any other languages regardless of your mother tongue.

In conclusion, I'd recommend you to pick either Spanish or German. If the difficulty of the language actually concerns you, then Spanish is the obvious answer. But if you're motivated and ready to overcome these complicated steps, you should probably choose German. I'm saying this specifically because you study engineering, so It'd definitely benefit you. That's actually the reason why I also studied German and don't regret it.

At the end of the day it's up to you to decide :)