Continuing on a different point while hoping for answer:
Are you familiar with Duplicati’s storage use? It only backs up changes, so use grows slowly.
This also means that a delete of a version might free up only the changes unique to that one.
Compare about backup sizes of different versions? is technical discussions, and led to a tool:
Duplicati BackupExplorer which I haven’t used, but it might help answer question of data use.
At some point you might just need more disk space, which is one way to also assist compact.
Compact - Limited / Partial (if done) might be another way to limit the compact space needed. Previously this was sought after more to limit time, but now there’s also space to worry about.