legacy .NET to .NET Core (which was renamed once again to .NET)  

It was always .NET, only that new one had 1 till 4 had additional "Core" to clarify any confusion that could come from having same numbers as old.

  here's .NET's simplistic garbage collector ... it tries to be a one-fit-all solution that basically cannot be tweaked at all  

Definitely tweaking GC is not a thing in .NET land but it is far from "cannot be tweaked at all".