Windows – Chrome Remote Desktop with Windows 10 headless NAS, No Physical Mouse attached, Cannot see mouse pointer, Mouse emulator

It is not possible. When connecting remotely using RDP, Windows enables some remote smartcard services which disables local smartcards until RDP client supports smart card redirection (at least for most smart card readers). Windows Remote Desktop for Mac does not support this.

You might be successful using rdesktop which can make use of smartcards. You will find it in all major package managers like homebrew, macports and fink.

I've checked all third-party RDP clients I could find, none were supporting smart card redirection.

Only possibility: Only connect using VNC or other software - I am using Teamviewer for this purpose. Not sure about whether someone else may be connected using RDP at the same time you connect.

You can't do this as standard with the desktop operating systems as they are specifically configured to be single user systems. You can do it with the server versions of Windows but that's a bit drastic (and expensive for home use). As you say, there are hacks about to give multiuser functionality, but these do have licence implications, and there used to be some boards and apps that allowed multi-user operation but I haven't come across any recently.