It will be tough. Spanish engineers are pretty good (I taught ESL to them while I was in college) but unfortunately most USA employers don't know that. You will be interviewing with people who have only been to Barcelona to party and do drugs. Many of people here in NYC are now from Ohio, Oklahoma and Colorado. These people have never worked abroad and especially not in Europe like I have. So they will look at the European degree and not know what to make of it or compare it to.
The only degrees valued in NY from elsewhere is maybe UK degrees, specifically from London. I am one of the rare well traveled Native NYers left in this city. The majority here are transplants and they've been to Mexico and that's it. There will be cultural barriers to get over, although your English is very good (likely due to studying in the Netherlands)
Even educated workers here are pretty dumb about the EU in general. They think Netherlands they think Marijuana shops and other irrelevant crap.
PM me if you want to discuss more as I have an opinion on this backed by direct experience and observation, but you will want to think very carefully about moving here and getting married to an American citizen. Many Spaniards I've known in the past do not like NYC for the longer term. Compared to Madrid, maybe Queens is where many are comfortable. I am not referring to trivial things either like siesta but more serious things like crime, population density, manners, cost of living (in Madrid you can more easily find cheap apartments) and things to do (in Madrid there are more parks, vegetation and places to explore in my opinion - But I'm biased as a Native NYer).
When a Man moves for a Woman in general there is more risk. The Woman will say she is comfortable being the rock and helping you get on your feet but I find that to be false. They will say one thing and do another. This is universal and not nationality specific most times. However, one thing I did not like for example Spaniard Women in the past I've dated were always progressive in public and traditional behind closed doors. Meaning they were a strong and independent Woman at work but at home demanded the Man "take charge" and pay more bills. Now the younger ones at the time (this was 2007) you would think at age 18-21 would be truly more progressive but they were not when you really got to talking to them.
It's similar here. Your fiancee may act like she's OK but you will find the labor market here to be very competitive even for Engineering, and you'll have to battle stereotypes or lack of knowledge on European credentials. They will also try to pay you a lower wage sometimes depending on the company because of the foreign credentials/origin. This is illegal but difficult to prove in court, of course.
I would highly suggest taking a step back, doing a lot more research and not rushing things. Especially because consulates abroad have now reduced funding it may take longer to do basic things under this "Trumphiant rule".
Can your fiancee move to Europe instead?
Most relationships I see succeeding the Man is in control. You don't want to give up your good job making good money only to be under-appreciated here. When my colleagues and I interview here they are very judgmental about your LinkedIN Profile, University and connections. I had one colleague rejected for a role he was perfectly qualified for because his LinkedIN profile was not up to date. The recruiter was a buffoon and submitted his candidacy without confirming with him or me - Instead of asking for a resume in confidence (many people cannot put full details on their LinkedIN profile while currently employed as it can contain information competitors in the industry can use to their advantage)
The recruiters here are stupid. And now they are outsourcing the recruitment for many NY jobs (whether tech, media or other) to Africans, Indonesians, Ukrainians and India. These people have never been to NYC and have no idea what a good candidate for the job looks like. Even if their English is good enough, they don't get the work culture here or have a solid understanding of the fields they are recruiting for at an American proficiency level.
In short, I don't recommend NYC because it's currently a clusterf***. Nobody knows what they're doing and everyone is interviewing but nobody is seriously hiring.
Except if you want to work at the burger shack for $15/hr with no health benefits
There are also legal considerations. I'm not sure if your wife is familiar but there is something called an "Affadavit of support" which means if you get divorced and use USA public benefits (such as food stamps or subsidized housing for poor people) she has to pay the government. She will be on the hook for this until you get roughly 10 years of work credit which means you've paid into Social Security and essentially you're paying for your own "safety net" (but not really, most single Men don't qualify for public benefits).
As you may be able to tell by statistics - Most Women are not comfortable being the provider for a Man or being on the hook in case things go south. She may even be required to pay you alimony - The attorney if he's a good one will go over all of this during the process. With my foreign Wife she was working here already for 10 years and I was able to have the exemption for Affadavit of support. We also signed a prenup as basically it's a wash. She has lower income but more savings, I have higher income but more debt. We both said we both walk away with nothing extra, just whatever is in our individual bank accounts. (Still no shared bank account to this day)
So I have experience in "international marriage" and have observed many fellow relationships over the years and seen which ones failed and succeeded. The ones that are still working the Man usually makes more money and has a more stable job. Despite what you hear on the news or in politics (both in Spain and USA) gender relations remain similar to 100+ years ago.