That is unfortunate ... everything looks perfectly fine. 😣
Do you have access to a Mac that runs a more recent macOS - just to test and see if this is an issue with your SD card or your system?
p.s.
Since you have a full backup already (with Linux partition maximized), we can do some test with that, which will be MUCH faster.
Maybe we can learn something here from the debug info without you having to run a 1-3 hour backup 😉
1. Remove SD cards and USB disks - to avoid confusion.
2. Start ApplePi-Baker - you should not be able to select any disks.
3. Double click the IMG file you made earlier (unzipped 32Gb IMG).
This will make it that macOS will try to mount the disks. Normally only the boot (FAT32) partition would show in Finder.
If you have a tool like Paragon ExtFS installed, then the Linux partition will be mounted as well.
Apple Pi Baker will report that these have been mounted, and you should be able to select the mounted disk now.
4. Enable the shrink option in APB, and start the backup.
Not sure if you run off an SSD or HDD, and how fast your computer is, but my Mac make a 4Gb backup (incl resizing) in 31 seconds.
Now look in the log Window, where you should see something like this (I think you may have looked at it before).
You can right click the log window and select "Copy Log to Clipboard" and paste it in a reply.
16:19:51 Backup -- START BACKUP
16:19:57 Backup Started - Making Disk to File Backup
16:19:57 Source - Disk: /dev/disk2 (4 GB Apple Disk Image) (3,623,878,656 bytes)
16:19:57 Destination - File: /Users/hans/Downloads/ApplePiBakerTests/backup.img
16:19:57 Shrink IMG Enabled - Temporary IMG file: "backup.tmp"
16:19:58 Backup started
16:20:05 Completed - Cloning Completed
16:20:05 Finished - Completed in 7 seconds, average speed 514.7 MB/sec
16:20:05 Resizing - Attempting IMG shrinking
16:20:05 RESIZE - Attempting to MINIMIZE a Linux partition in the IMG file "backup.tmp"
16:20:05 RESIZE - Find Ext2/3/4 partition and determine size and location
16:20:05 RESIZE - STATS:
16:20:05 File Size : 3,623,878,656 Bytes (7,077,888 Sectors)
16:20:05 Linux Partition : 3,351,248,896 Bytes (6,545,408 Sectors)
16:20:05 Starts at : Byte 272,629,760 (Sector 532,480)
16:20:05 Linux Target Size : Minimum size
16:20:05 RESIZE - Extracting Ext2/3/4 Linux partition
16:20:12 RESIZE - Verifying and Fixing extracted Linux Partition
16:20:12 RESIZE - Shrinking Linux Partition to Minimum Size (double pass)
16:20:13 RESIZE - Creating new IMG file and Updating Partition Table IMG file
16:20:21 RESIZE - Shrinking of IMG file completed
16:20:21 Original:
16:20:21 - IMG File Size : 3,623,878,656 Bytes
16:20:21 - Linux Partition : 3,351,248,896 Bytes
16:20:21 Resized:
16:20:21 - IMG File Size : 3,623,878,656 Bytes
16:20:21 - Linux Partition : 3,351,248,896 Bytes
16:20:21 Bummer ... File size did not change in size by resizing the partition
16:20:22 Backup - Backup and Resizing Completed in 24 seconds
16:20:22 Backup -- END BACKUP