You can list all your backups using the following command:
The you delete some some backups from the list e.g.:
sudo tmutil delete /Volumes/Time\ Machine\ Backups/Backups.backupdb/yourmachine/date
It will report the size of each backup deleted and total if deleting more than one at a time. Once you have deleted sufficient backups you may ...
Describe me as a minimalist, yet I dropped most of GUI solutions and by experience, I prefer to use the following strategy:
fire up you command-line interface client (terminal, iterm, ...)
identify the path of the drive that is full: /, /Volumes/data and change to this directory (that is; cd \)
ask for the size in the hierarchy and sort it: du -k |sort -n
I copy the contents of the container to a new disk / volume and then evaluate if it's faster to erase the entire drive or I want to merge / rearrange things.
The short answer is the tools are designed to have one container for the entire storage so you can move date between volumes without running into ordering issues. One container per drive is how I run ...