Another country? I.e. in your 40s, taking a company relocation may be the best bet?
Marrying between cultures is kind of a different thing since you aren't seeing each other in the same sets of social layers and translation difficulties ultimately simplify unnecessary rituals that might trip up two native speakers.
There are also more blunt approaches like finding an immigration orientated spouse and that can work if they are really on the same page on the lifestyle with children, etc.
As someone who stumbled into a long term relationship with an immigrant of a very different race and culture this is probably the last thing I would recommend to an middle-aged person, especially one wanting to have kids. I know that might be hard to believe if you've looked at my profile/post history but I used to be very liberal and believed in the whole "race doesn't matter" thing. The enormous amount of propaganda and taboo around this means you're left figuring out what it means more or less on your own, this took me years. If there's any place where race (and culture) matters it's when you're deciding who to have a kid with.
Also USCIS is an absolute fucking nightmare. Imagine the worst caricature you've seen of a bureaucracy, this will top it. It's a bureaucracy with an effectively impossible mission and almost no accountability (at least on the end you'll be caring about.) Every time you interact with them there's a decent chance something goes wrong (maybe it's their fault, they're extremely incompetent, maybe it's your fault, the rules are very complex, maybe you just get unlucky) and you're left with the choice of staying illegaly or leaving the country quickly.
Domestic matters are inherently too complex to predict. Through my life I've known many long term happy couples with substantial cultural differences and seen a lot of divorced in ~5 years couples from similar backgrounds.
The US is definitely a more difficult place for the couple to be settling, though I know some there. I find European countries are simpler logistically in an absolute sense (not necessarily familiar to an American) and the average European who marries someone from a different continent is starting from more knowledge of intercultural complexity than someone in the US who doesn't really need much more than shallow awareness of other cultures.
race doesn't matter. culture does. they are often aligned, but not always. different cultures can work if both partners are open and willing to compromise.
older people can be more mature about it but also less easy to adapt.
Marrying between cultures is kind of a different thing since you aren't seeing each other in the same sets of social layers and translation difficulties ultimately simplify unnecessary rituals that might trip up two native speakers.
There are also more blunt approaches like finding an immigration orientated spouse and that can work if they are really on the same page on the lifestyle with children, etc.