EliteDesk 705 G4 SFF won't boot - 3 long, 3 short - even with 'known good' graphics card

[USER20222710213]

Wondering if anyone else has encountered/resolved this issue:

After disabling Secure Boot in the system BIOS for an EliteDesk 705 G4 SFF, I noticed that there seemed to be some additional image cache remaining that seemed safe to remove, so I opted to 'purge' what remained - while at the same time, I thought I'd also uncheck a box related to 'USBCI' in an effort to resolve 'usbci_aspci': PPM init failed (-110)' errors which were preventing the Linux OS from booting (I know, too many variables,..) Shortly following those two (or three if we include the software/driver upgrade step for the AMD Radeon 520 graphics card which basically brought all this on eventually) the system stubbornly refuses to boot  or even fully POST....I can see the LEDs  light up, and I see that the cpu fan is running...but the same beep codes happen in sequence each time: 3 long beeps followed by 3 short beeps. I've tried pulling the battery from the CMOS for an hour, just about every combination of keyboard-reach + power-cycle I've seen suggested, and yet it just looks to be a total loss (which just shouldn't be a form of lurking peril each time there's a need to enter the bios to perform a legitimate action..)

Based on what HP has documented , this typically means that a problem exists related to the graphics card - so I tried plugging in another 'known good' graphics card from another near identical Linux host, and it it failed in exactly the same way....I'd typically walk away from such a quagmire - but to only have a system for 3 months before it becomes useless? At the very least  I just want to know how pi55ed I'm allowed to be at HP - and then wallow in whatever ultimately inconsequential  but satisfyingly self-righteous decision I make to 'take my business elsewhere!'  thanks 🙂