Great advice Michelle,
The regional accents and wide variance in nouns, plus slang, make going country to country bit of a challenge.

Language wise I liked Colombia. Generally more well educated folk can be a bit easier to understand vs. native Aymara speakers in Peruvian Altiplano. For them ... Spanish may be their 2nd language!

Experts claim Colombian Spanish is the very best outside of Spain. I agree.

Getting beyond simple Gringo level Spanish can take work. For some it comes easy, others struggle. I do good at speaking ... but understanding for me is sometimes hard ... especially if it's a weird accent like Cuban or Chilean and they hear me speak (I have a perfect accent) and think I'm fluent (far from it!), the they take off speaking a thousand miles an hour ...
and I'm sunk.

I understand Argentine Spanish well because I spent years there off and on. Parts of Mexico, I hardly get a word with some of the "country" dialects you find.

Take a course if you can, should be plenty of them in Colombia, Ecuador and parts of Peru' too. Really helpful, but self study everyday is best ... then get out and USE your Spanish everyday, engage folk in conversation. That is the way forward.

If you can learn about 8 survival Verbs, especially SER and ESTAR, the two verbs TO BE, in all there forms ... you'll do just fine. A few key pro nouns and common usages and responses, you'll do OK.

I think you may find more and more young people in S. America speak some English. Resist switching back to English for them, stay in Spanish and practice on them, don't let them practice on you! (English gets you no where language wise)

English is taught in most all schools starting at grade school level ... now add
English language TV shows/movies ... you get the idea.

I helped teach English in El Salvador years ago. Just for one semester, once a week. Poor teacher struggled with pronunciation, so I was glad to help correct the students ... some movie watchers knew more than the Maestra. Great result for the kids! I'm sure they're all living in California now. (they all wanted to move there!)