Thanks Guys for the clarification. I'm going to try a couple of more things before going that route... meanwhile, I found this on the inter webs to produced a TM log file: log stream --predicate 'subsystem == "com.apple.TimeMachine"' --info --debug
Starting manual backup
Attempting to mount 'smb://TM_MacPro@timemachine._smb._tcp.local./TM_MacPro_SMB'
Mounted 'smb://TM_MacPro@timemachine._smb._tcp.local./TM_MacPro_SMB' at '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB'
Initial network volume parameters for 'TM_MacPro_SMB' {disablePrimaryReconnect: 0, disableSecondaryReconnect: 0, reconnectTimeOut: 60, QoS: 0x0, attributes: 0x1C}
Configured network volume parameters for 'TM_MacPro_SMB' {disablePrimaryReconnect: 0, disableSecondaryReconnect: 0, reconnectTimeOut: 30, QoS: 0x20, attributes: 0x1C}
Found matching sparsebundle 'MacPro.sparsebundle' with host UUID 'D2487E77-1697-5776-BE5A-28C60EEFC234' and MAC address '(null)'
Not performing periodic backup verification: not needed for an APFS sparsebundle
MacPro.sparsebundle' does not need resizing - current logical size is 3.8 TB (3,798,891,797,504 bytes), size limit is 3.8 TB (3,798,891,797,913 bytes)
Mountpoint '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB' is still valid
Checking for runtime corruption on '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB/MacPro.sparsebundle'
Mountpoint '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB' is still valid
Runtime corruption check passed for '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB/MacPro.sparsebundle'
Stopping backup because volume '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB' was unmounted.
Backup cancel was requested.
Failed to attach to '/Volumes/.timemachine/timemachine._smb._tcp.local./0F0C58C8-0505-4FDE-BB64-4FF393375FB7/TM_MacPro_SMB/MacPro.sparsebundle', error: 112 no mountable file systems
Waiting 60 seconds and trying again.
Backup reporting that it needs to be cancelled
Backup canceled (22: BACKUP_CANCELED)
looks like the TM share mounts ok, then gets knocked offline for some unknown reason... thoughts?