I had some time to futz with this again, updates to 14.1.1 but could not get File Sharing to work by turning File Sharing off, rebooting the Mac (and router)then turning on File Sharing again.
As per this article, SMB File Sharing is supposed to happen on port 139.
TCP and UDP ports used by Apple software products - Apple Support
With Stealth Mode disabled in Firewall, I'm noticing Port 139 is not even listening.
Open TCP Port: 88 kerberos
Open TCP Port: 445 microsoft-ds
Open TCP Port: 625 dec_dlm
Open TCP Port: 3689 daap
Open TCP Port: 5000 commplex-main
Open TCP Port: 5900 rfb
Open TCP Port: 7000 afs3-fileserver
Open TCP Port: 49177
Open TCP Port: 49733
Port Scan has completed…
I opened up ports 135-139 on my Router (port forwarding to my Mac) but that did to bake any difference. To me, it looks like there's problem with the port that handles SMB transactions. Im getting Authentication errors when attempting to connect too server from the Files App (on iPad and iPhone) and this would explain that. If it can't connect to authenticate, well, that would explain it as that's the first thing in any session.
I tried nc -zv iMac.local 139 from he command line (stealth mode still disabled on Firewall) and returned this,
nc: connectx to iMac.local port 139 (tcp) failed: Connection refused
nc: connectx to iMac.local port 139 (tcp) failed: Connection refused
nc: connectx to iMac.local port 139 (tcp) failed: Connection refused
nc: connectx to iMac.local port 139 (tcp) failed: Connection refused
So. clearly, there is an issue with the SMB port. Still stumped....