How to fix Time Machine 'Preparing backup' issue when Spotlight indexing cannot be disabled?

As VikingOSX has already mentioned, Time Machine (TM) depends on Spotlight Indexing.

Regardless, when trying to ascertain a "tricky" TM backup issue, the best way to figure out where the culprit(s) is/are, is to review the macOS system logs.

On Catalina, this TM phase commonly stalls due to filesystem snapshot issues, permission errors, or a specific path that TM repeatedly fails to scan. These don’t always surface in the UI, but they do show up clearly in system logs.

If you're game into tackling the logs, here’s what I suggest you do next:

• Open Console.app and filter for `backupd` and `TimeMachine` while a backup is stuck in “Preparing.”
• Look specifically for repeating errors such as:
  • Failed to scan volume
  • Error enumerating files
  • snapshot invalid or com.apple.TimeMachine.localsnapshots
• If a specific path or folder is mentioned repeatedly, exclude that path from Time Machine (not Spotlight) and retry the backup.
• If the logs point to local snapshots, enter the following two commands in the Terminal app to both list all current local snapshots that are present, and to remove them. This safely clears problematic local snapshots, which is a very common cause of endless preparation on older Macs running Catalina.
  • tmutil listlocalsnapshots /
  • tmutil thinlocalsnapshots / 9999999999 4
• Finally, if logs show permission or ownership errors, a full-disk permission reset (via Recovery → Disk Utility → First Aid on the internal disk) is worth doing again after snapshot cleanup. At that point, if Time Machine still hangs with clean logs, the issue is almost always OS-level corruption, and recreating the Time Machine destination (erase + reselect) becomes the most reliable fix—even when the drive itself is healthy.

I'm throwing a lot at you, so if you need further assistance with any of these steps, please post back.