Make sure to use the gupdate.gcd from the "System_Backdate" folder. (Only those have the "Downgrade"-flag set and I'm pretty sure that's needed.) The one for the fenix 5X Plus should be the same as the one for the fenix 5 Plus. The firmwares (and signatures) for D2 Delta and D2 Delta PX (also fenix 5 Plus and 5X Plus) are exactly the same - they only have different SensorHub firmwares which is installed separately.

When the watch asks to install the update, maybe keep the START button depressed for a minute or two and see if that makes a difference.

I could also send you the file I've used. Should work for your PX as well.