First, SQL Server and SQL Server Management Studio (SSMS) are separate programs and installs. In your case, it is SSMS that is misbehaving, so there is no need to meddle with your SQL Server installation.
I can't say why SSMS is closing silently, but SSMS is a very complex product, and it is not unheard of users having problems with some parts of SSMS.
What you could try is to download and install SSMS 18.12.1, an older version that uses an older version of the Visual Studio Shell. Note that you do not need to uninstall SSMS 20, since different major versions of SSMS are installed in parallel.
If SSMS 18 also fails, you could try Azure Data Studio, which is a leaner tool, but which also has less dependencies. It does not seem to have a flat-file import, but it has an Import wizard.