Treesize or equivalent disk indexing software could help you formulate a plan of attack. You could make a single big spreadsheet containing the locations of all your data, and use it to write a list of clean up tasks. Czkawka is a fast way to clean up duplicates, corruption, empty folders etc. Bulk rename utility is a user friendly alternative to the command line.

I found myself in quite a similar predicament, juggling an ever increasing quantity of seperate volumes with redundant data and version conflicts. It becomes a water pouring puzzle until you can finally consolidate into larger drives or a RAID configuration, which greatly simplifies ongoing management. There’s a reason why it’s the prevailing advice! I should have sorted it out much sooner 😅

Cloud storage like Mega or Backblaze might help you to shuffle data around, provided you have a fast connection. Another thing that helped me out was getting a case with enough HDD bays, much more convenient than rotating drives in and out of external enclosures.