What I'm attempting to get across is that oligarchies by themselves aren't inherently any worse than other forms of rule. Corruption is what makes them and other forms bad.
There is no contradiction between being a republic and a democracy unless one unreasonably restricts the definition of democracy to direct democracy. Reasonable definitions of democracy include representative democracy, which is practiced in many republics.
As we see, many republics face problems with all of the above. Including US. Technicalities such as non-majority presidents and strict bipartisan systems...
In certain countries like UK, where apparently ruling party was against the strict referendum results.
In Poland, where the ruling party is changing the judicial branch to their benefit.
Those are not third world banana republics.