I'm not sure how to find the best cities to invest in but am leaning towards DE as I spent time there and have a little experience with the state. 

I have traveled all over Mexico and one of the places you did not mention was San Miguel de Allende.

Other than that, mistakes related to expenses are common, and not electing things like the De Minimis Safe Harbor election, which can save you if you get audited.An option you might want to consider if you do your own taxes is at least have a tax professional review your return after you complete it (but ideally before you file it). 

In Delaware a 5 day grace period is mandatory so you can't collect a late fee until the 6th. 

I have a 23 unit condo building in Chicago, and I currently own 22 out of the 23 units with the intention of buying out the last unit. I have heard from some people, but have not gotten a definitive answer, that going...

Quote from @Steven De Lange: BRRR out-of-State is a great strategy if you have a good team of people in the area you are looking to invest (realtor, property manager, and contractor/ project manager)- a team you can build through BP. 

Since the owner would be paying for electricity, I would consider looking into adding a solar option for certain buyers.

Prices skyrocketed from massive government actions and those actions first stimulated demand and later de-stimulated supply.People are not buying because they have more knowledge now...it's actually the opposite, transactions are at record lows.

I own a single-property home in Delaware.

Would any investors (especially those in short-term rentals in Wilmington, DE and Pittsburgh, PA) be willing to chat?