It seems that SEA fw_all.bin actually boots normally, as it's able to enter mass storage mode and update GarminDevice.xml.
I think SEA fw_all.bin checks for HWID and UnitID and displays "SYSTEM SOFTWARE MISSING" in case of mismatch.
Try using the attached fw_all.bin v2.40 with the above check disabled. Use it as usual, i.e. change HWID with RGN_Tool and build the cross-flashing RGN.
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