Free space of toasted content
A plain VACUUM will make the free space available for new data (i.e., other TOASTed JSON data).

If you really want to give the free space back to the OS, and you cannot afford the exclusive lock, try pg_repack:


  To perform a full-table repack, pg_repack will:
  
  
  create a log table to record changes made to the original table
  add a trigger onto the original table, logging INSERTs, UPDATEs and DELETEs into our log table
  create a new table containing all the rows in the old table
  build indexes on this new table
  apply all changes which have accrued in the log table to the new table
  swap the tables, including indexes and toast tables, using the system catalogs
  drop the original table
  


It still needs enough space for the original table and the new copy.