PyWinrm Persistent PowerShell Sessions
You should use pypsrp, which uses PowerShell remoting natively.
The examples in the GitHub repo will show you how, and you can continually use .add_script() or add_cmdlet() to make multiple calls within your session.
WinRM (and by extension pywinrm) is not strictly related to PowerShell, so that call you're doing likely just opens powershell.exe with whatever you send it.
PSRP is a separate protocol, which runs over WinRM as its transport, so you'll be connecting to a hosted PowerShell session.