If you're on Windows you can use Windows Powershell or Powershell 7 with Out-GridView to have a nice list of branches and select with mouse which one you want to delete:
git branch --format "%(refname:short)" --merged | Out-GridView -PassThru | % { git branch -d $_ }
after clicking OK Powershell will pass this branches names to
git branch -d
command and delete them