GitHub - Freaky/Compactor: A user interface for Windows 10 filesystem compression
Compactor can help compacting these ROMs using Microsoft's own COMPACT.EXE command line tool and the LZX (most compression) format. Needs Windows 10/11. You can compress almost anything in your PC using Compactor/COMPACT.EXE and Windows 10/11 will access, read and write it transparently, as if it wasn't compacted. If you modify a file compacted with these COMPACT.EXE methods, the file will be uncompressed and you must compress it again manually.
It's a very decent tool, but it's nowhere near the highest compression methods you have out there, like LZMA2. But it's better than uncompressed, and it's transparent to Windows and RPCS3.
EDIT: my main concern in using a container is that those 12,511 files from Demon's Souls would be contained in a single file (which is better than 12,511 files).
EDIT 2: PS3 folder (46,938 files, 1548 folders):
Uncompressed: 713GB
LZX Compression: 668GB