Unfortunately, block devices with file systems on them tend to not get smaller. Unlinked files leaves data in free blocks, which even if compressed still take up space in an image of the entire disk.
Try trimming free space on block devices. fstrim -va once, and note if mounted volumes trimmed or failed. If successful, enable a weekly or so background job, like fstrim.timer that probably exists on systemd Linux.
Or, restoring from backup is an opportunity to only copy used space. Hopefully leaving unused blocked as easy to compress zeros. As what you have is another block device, look at file system aware tools like xfs_copy or e2image -rap that do not copy empty blocks. Create a new disk (starting zeroed), take the system down, and restore from the backup. Bonus, might count towards on occasional backup restore test in the business continuity plan.
Or, when the files using the space are still around, various "secure remove" utilities can be run in a mode that zeros then unlinks them. Just zeroing, because writing all over a SSD many times is not great for its wear. GNU coreutils can do zero and unlink: shred -v -n 0 --zero --remove=unlink --
Although all this is a lot of work to save a mere 150 GB. Your protection storage used for backup should have several times the size of the primary disks, to keep backups around for the retention you require. A safe thing to do is just let the backups be ~300 GB.
