I couldn't find an answer anywhere that worked for me, and I figured this out so I wanted to share. I have a spfx listview command bar extension as well and it's to replace the New/Edit buttons on a list with PowerApps deeplinks which I'm sure is a growing need. Seems to be what OP is looking for.
The "Key" is to add SpfxCustomActionNavigationCommand_ as a prefix to your custom button keys as they're defined in your spfx code. Then you can do whatever you want with them like normal command bar buttons in the list view JSON formatting. If you want to replace the New button you can use the Primary property but it must be in postion 0 for it to assume the Primary button style. Also hide the new button as well of course. As your needs determine etc. See mine below:
My custom button keys are COMMAND_1_MTR_NEW and COMMAND_2_MTR_EDIT, but I had to add the prefix for it to work, this is a cut out I'm assuming you have the schema and are hiding buttons or doing other thigns above this:
{
"key": "SpfxCustomActionNavigationCommand_COMMAND_1_MTR_NEW",
"position": 0,
"primary": "true",
"text": "Create New Transfer Request",
"iconName": "Add"
},
{
"key": "SpfxCustomActionNavigationCommand_COMMAND_2_MTR_EDIT",
"position": 1
}