Difficulty inheriting Time Machine backups via old MacBook Pro set up to be a wireless server for USB backup drive

I'm having difficulty getting a Time Machine backup external drive to now function via a old MacBook Pro I'm setting up as wireless server. Goal: to be able to wirelessly do Time Machine backup from the 2 others MacBook Pros we use.

Details of former setup:

• Sandisk 8TB GDrive was being manually connected by USB-C cable to one of our MacBook Pros and then the other, to do Time Machine backups, about 1x a week. Also had all files being backed up on Google Drive. The GDrive is set up with 2 partitions for the respective Time Machine backups, both 1.5 TB APFS formatted, plus a 5TB partition for other general storage, formatted MacOS Extended (Journaled).
• My wife's MBP - now replaced with a newer one - is what I'm now using at a wireless server. It is an early 2015, running Monterey 12.7.6 - completely wiped and set up fresh - I was thinking of selling it.
• My MBP - also replaced now - was a mid-2017 running Ventura (13).
• Both of these were doing fine with their Time Machine Backups

Current attempted setup:

• Backup drive: same Sandisk 8TB GDrive with same partitions, connected by hi-speed USB cable to the server.
• Server: early 2015 MBP running Monterey 12.7.6, and connected to router by ethernet cable via thunderbolt adaptor. Has a main account and a 2nd (if needed for emergency) administrator account
  • Sharing: File Sharing is on; All three partitions from the GDrive are listed as shared folders; All three have read...write selected for main user, staff, and for everyone. Under options, both main & 2nd accounts have SMB sharing enabled - not sure that was needed, but trying it anyway.
  • Also have Remote Management sharing turned on.
• My new-to-me MBP is a 2020, running Tahoe 26.2. My wife's new MBP is also running Tahoe 26.2. We did inherit the Time Machine backups to our new MBPs by manually attaching to the GDrive via USB, and so that is working - but still terribly inconvenient.
• On my MBP, in the Time Machine window, I can see my Backup (which I can use by manually connecting via USB to the GDrive).
• When I hit the + button to add a drive (I think this is the correct path), I still get this message: "The selected network backup disk does not allow reading, writing and appending. Please connect as a different user ... "

So I'm stuck. Help! Any wisdom about what I should do to get these backups working again remotely via the server?

Thanks!