OK, no answer, so I hope my own solution will help:
#!/bin/bash
value=80 #Disk % threshold - if disk full above, then oldest snapshots will be deleted until disk is below threshold
for i in 1 2 3 4 5 6 7 8 9
do
echo Attempt: $i of 9
echo Getting snapshot list...
to=$(sudo btrfs subvol list /mnt/timeshift/backup/timeshift-btrfs/snapshots)
search="snapshots"
prefix=${to%%$search*}
position=$(awk -v a="$to" -v b=$search 'BEGIN{print index(a,b)}')
fn=$(echo $to | cut -c $(($position+10))-$(($position+28)))
echo Oldest snapshot name: $fn
diskfull=$(df -hP / | awk '{print $5}' |tail -1|sed 's/%$//g')
if [ $value -gt $diskfull ]; then
echo Threshold $value% greater than Used disk $diskfull%
echo should not delete any more snapshots
echo Creating one more snapshot
sudo timeshift --create --comments "automatic"
exit 1
else
echo Threshold $value% is less than Used disk $diskfull%
echo Deleting snapshot $fn...
sudo timeshift --delete --snapshot "$fn"
echo Waiting for changes to be written...
sudo btrfs subvolume sync /
echo Checking if disk space resolved...
fi
done