iPhone backup using Symlink has stopped working

The symbolic link process has never worked for me. Though for your benefit I have given the process at the end of this post. The risk is all yours.

I would recommend you make space, take the backup and then copy to another location and delete it from the default place.

Delete or copy backups on your Mac or PC

Don't edit, relocate, rename, or extract content from your backup files, because these actions might ruin the files. Use the following sections to make copies of your backups or delete backups that you no longer need.

Copy backups to another location

If you need copies of your backups, you can find the Backup folder and copy the entire folder (not just part of it) to another place, like an external drive or network-storage location.

When you use Time Machine, it automatically copies your Backup folder when you back up the Home folder on your Mac.

Delete backups

1. On a Mac with macOS Catalina 10.15 or later, open the Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes. Click your device. 
2. In the Finder, under the General tab, click Manage Backups to see a list of your backups. From here, you can right-click on the backup that you want, then select Delete or Archive. In iTunes, choose Preferences, then click Devices. From here, you can right-click on the backup that you want, then choose Delete or Archive. Click OK when you're finished.
3. Click Delete Backup, then confirm.
4. 

If you still feel you need to process and try it out, it will be at your own risk...

You can but you need to be well-versed with the Terminal app. If you mess up you may never be able to back up again even on your Mac/PC.

You will have to create a symbolic link to the backup. See the Step 3 process on --> https://www.imore.com/how-move-your-iphone-or-ipad-backups-external-hard-drive

Please follow SravnKrA’s suggestion. If it doesn’t work it might be because a vulnerability in the backup process was reported that would allow someone who hacked your computer to also access the contents of your iPhone backup, and Apple took action to defeat the vulnerability. If the backup is in its default location it is protected, but not if it is located anywhere else in the storage address space of the computer. Here is a summary of the vulnerability:

Available for: iPhone 8 and later, iPad Pro (all models), iPad Air 3rd generation and later, iPad 5th generation and later, iPad mini 5th generation and later

Impact: An app may be able to access iOS backups

Description: A permissions issue was addressed with additional restrictions. 

CVE-2022-32929: Csaba Fitzl (@theevilbit) of Offensive Security

If you want details you can find them in these two links:

• From the discoverer of the vulnerability→CVE-2022-32929 - Bypass iOS backup's TCC protection · theevilbit blog
• From a disinterested 3rd party→iOS Backup Passcode Prompt-iMazing