For what it's worth, there's a firmware upgrade to A.03.07 on the HP website.
Also, the first byte of the error codes indicates the component/system.
63: network i/o card
79: firmware error (suggestion is to upgrade to latest firmware).
61: language interpreting (postscript or hpgl+ error)
86: carriage movement
The service manual is easily had via google if you're curious.
If your printer does not have an HPGL2 card, then it's memory card cannot be upgraded. IMHO, that's fine. I've found my workstation to almost always rasterize loads faster than a printer's embedded CPU.
Also, the JetDirect card in the back (the network interface) may well have/need/could use a firmware upgrade as well.
Gorgeous find! Thank you for all the videos.