A GPA of 3.6 with a dual bachelor in maths and mech. eng will get you into practically any grad school in Europe.

Going back to school is probably the most straightforward strategy, for reasons that have already been pointed out earlier in this thread, especially network, language skills and applications for permanent resident status or citizenship. The french law is particularly accommodating in this regard: you can apply for citizenship after having successfully completed 2 years of higher education and can demonstrate sufficient proficiency in french

Regarding learning a language, it can be harder to learn dutch or danish, because everybody speaks english so perfectly (in my experience), so you don’t really have any incentive to learn it. Generally speaking you shouldn’t consider language to be a barrier, at least as long as you stay in cosmopolitan cities.

If you have 80k in your college fund I say go for the masters. With that much money you can live more than decently for the 2 years of your masters, even in the most expensive cities in Europe such as Zürich, Paris or Copenhagen, and still have 20 to 30k left at the end.

Don’t hesitate to pm me if you need advices on universities