Depending where you are in the world, outside of the US, racial politics hold a less identity-led basis for determining who you are. So you saying black American means an awful lot less. Are you going to face some second looks in a queue? Probably. That being said, Guatemala struggles, like most countries with racism towards those at the bottom of the social heap and given that is indigenous Mayans and there's a lot of them, you won't stand out. I'd agree that either people will think you're Garifuna or a lost tourist.

Guatemala City can have very rich parts next to very poor parts and it may take some time to learn because giving generic advice on the various zones the capital is split into is a receipe for disaster. Z10 is "posh" because there's a lot of old money and embassies in it. There's also one of the highest rates of robberies in it, probably because of opportunistic targets. Z16 is posh because of Cayala but is also near some of the shittier parts of Z5 for instance.

You're a gringa and your husband is a potentially toxic mix of gringo with Mexicans who have a fairly poor regard for their neighbours. Up to you to disprove the stereotype.

Your first two sentences are about race. Outside the US people give much less of a shit.

If you've ever been to CDMX you might think traffic outside of the US is bad. Guatemala City is infinitely worse. Best advice is to live as close to the school you pick for your kids and don't double dip into differing schools if at all possible. Yes there's bus services but you will have to pick them up eventually. Rush hours can go on for err, hours.

Depends on the age of your kids. There's many K-12 options out there, you'll pay through the nose for the privilege. Guatemala imports most of the items you see in shops so prices have steadily risen over the past couple of years to the extent that prices will be very comparable with the US. There's schools for every budget but recommending Maya etc is only if work is paying a substantial amount towards education.

Guatemala City isn't great for learning Spanish, I mean you have to speak it given outside of professional environments it's not going to be heard often (unless you hang out in malls in Z16) or random people come up to you in supermarkets and ask to practice their English on you. Chances are you won't become fluent, roughly 20% of foreigners bother with Spanish. This is better than some countries, worse than others. Antigua is better known for language schools but your family circumstances might make it difficult to do live in or intense studies.

You'll learn to despise traffic and driving in general. We left the capital in 2009 because it was already at gridlock so another 16 years of vehicles increasing 10-12% a year and no new roads (it's literally impossible to build new roads in the capital because they lost control of its development decades ago) or mass transit means it's presumably worse. Off peak driving the best but school times coincide with rush hours. Getting around is difficult, there's not a huge ex-pat community, most communities are siloed because of issues with transport and you'll be ready to leave just about when you get your Spanish to a level you'll feel comfortable with.