Just another voice.
I can confirm that it was my Sophos Antivirus, too. It works if you disable on-access scanning in Sophos, but it cannot be a final solution to be stuck without anti-virus software 😟.
I tried to exclude my Time Machine Backups in the Anti-Virus, but it did not help.
Just to mention: After disabling Sophos I had to reboot to make backups work again.
As for the backupd process: It was running between 95 and 99% CPU load most of the time when it was stuck in the "Preparing backup" stage. When the backup is working, the load on my machine is between 10 and 50%.
This just as a further indication: If your backupd process stays at full CPU load (95-99%) in the activity monitor, your backup stage will likely never leave the "Preparing Backup" stage.
Was struggling with this for days on and off before I found this, trying out dozens of different things, repairing the backup disk etc. ðŸ˜