Hello from Argentina, want to share my experience.
I have found plenty of places to change your dollars to pesos, so far I have change 2 x 100 USD note. When you approach the black market cambio person, make you he offers the correct blue exchange rate , you can check on here first Blue Dollar Rates : Dólar Blue Informal Rate of US Dollar in Argentina | Blue Dollar Rates .
My monzo has been accepted in most places including smaller shops and cafes. They are using the MEP rate which is just very tiny lower than the Blue rate. Saves you carrying all these pesos notes.
There has been a few times card not working, so I strongly recommend you carry some cash with you.
Carry cash for street purchases. Eg ice-cream van, hot dog stall, barbecue burgers stall and all these lovely Latin American street food.
Local taxi, you need cash for that. Uber is available but I found the prices is around the same and I rather used official taxi, they are on a meter so you don’t need to worry about being ripped off.
Ref Uber, for some reason in Argentina it does not accept foreign debit card payment, it was credit card only, this is where my Monzo flex card come in handy.
Safety. No issues at Buenos Aries, Cordoba and Salta. Just be careful and be alert at all time in crowded areas and public transport. Especially for BA, learn where are the no go area and avoid them. Wear your rucksack, bag etc at front of the body. Be very discreet with your cash and mobile phones as well. I be honest as I live at Croydon, I felt a bit more safe here, plenty of police around both federal and local, they are extremely friendly as well. Always happy to help.
This was my first trip to Argentina and South America and I am already planning to come back next year.