The actual firmware version you have 1.00.21 is I am fairly sure the latest - the custom firmware thread I pointed to earlier, would have been very busy if Humax had released another as it would have replaced it.
I would not worry about flashing the loader version U 7.54 (remember to rename as Luke advised above). You refreshed 7.51 successfully after all. There and several safeguards during the flashing process, checksum, version, system ID, name, etc.
Like Trev I have some doubts that it will make any difference.
However does housekeeping now work OK? You can tell if new recordings have thumbnails.