You could do a full backup every week or month using proxmox-backup-client, and combine it with a rsync proces that creates a separate folder with a copy of "changed files in the last X days", and made then also a backup of that using proxmox-backup-client.
So you will have 2 backups and select the most apropiate.
Then you can prune or delete differential (rsync created) copies, when a new full backup is done.
You can use something like that, to create a folder with a copy of files modified in the last day, that then you will backup much faster:
https://unix.stackexchange.com/questions/173958/find-files-newer-than-a-day-and-copy
export DAYS=1
export BASE_DIR=/mnt/data (sample, folder with lot of files and TBs that I full copied previously, and I want to copy incrementally)
export DATE_DIR=PBS_$(date "+%Y-%m-%d")
mkdir -p "$BASE_DIR/$DATE_DIR" (new folder that will contain only a copy of new files)
cd $BASE_DIR
find . -type f -mtime -$DAYS -not \( -path ./$DATE_DIR\* -prune \) -exec rsync --relative {} $BASE_DIR/$DATE_DIR \;
proxmox-backup-client backup sample.pxar:$BASE_DIR/$DATE_DIR --backup-id 'sample-incremental-1day'