Part of the .RPT produced using db2dart to attempt lowering the HWM:
High water mark: 18074938 pages, 9037469 extents (extents #0 - 9037468)
Lower high water mark processing - phase start.
Current high water mark: 9037468
Desired high water mark: 0
Number of used extents in tablespace: 4199670
Number of free extents below original HWM: 4837799
Number of free extents below desired HWM: 0
Number of free extents below current HWM: 4837799
Step #1: Object ID = 4062
After many many repeated attempts of following the suggested reorgs and reducing the tablespace, each attempt has minimal change in the HWM.