Reading all kind of CBDC propaganda and those who are clearly against CBDC I absolutely can't get something real from that stuff.
Every single feature/option/whatever of CBDC could be implemented using current bank card/account system without single problem. Everything, from any restrictions to programming buying power or whatever. Every single thing that somehow connected to CBDC in everything written about it and posed as threat/advantage by whatever side is a question of a few minutes scripting in bank computers over current card/account system.
So, what is the real point of pushing CBDC crap by TPTB? Money laundering? Creatiing a base for some other narrative unknown yet, like kind of cyber threat fearmongering? What else? Anybody?
Your last paragraph is exactly what I think they're doing. Shared blockchain, but different named CBDCs on the front end.
But it is senseless while there will be countries with cash. It will be relatively easy to just use the other country cash if your own country will get rid of it. There will appear a lot of gray/black exchanges where you "buy" something with CBDC in another country and immidiately get foreign cash from moneychanger. Then use that cash as you like. Another person who get that foreign cash could do the opposite - "do some service for a foreigner" along with giving cash to moneychanger and get CBDC.