Context: I am a software engineer living and working in the US. I am a US citizen and my wife is not, and we are working on getting her permanent residency.

It's definitely worth investigating the difficulty of immigrating to the various countries. Canada and Singapore both have reputations for being a lot easier for skilled workers to immigrate to than the US, but YMMV.

Note that if you do marry an American, be sure to start the process of getting your permanent residency while you're still in the country. My wife and I made the mistake of starting the process while she was temporarily abroad, and that has caused significant delays.

See if there are companies that are willing to hire you while you're on OPT. I think that buys you a couple years to try for an H1b or perhaps get married. If they're willing to hire you in the US and let you transfer internationally if immigration doesn't let you in, even better.

Also keep in mind that if you become a US citizen, the US will tax you on your worldwide income, not just your US income. That's not an issue if you plan on staying in one place, though.