Blackscreen, no chime, fan on, motherboard led on

I recently fixed this iMac with a new SSD, installed Linux and everything was fine.
Suddenly it just doesn’t start anymore.

Fan is spinning, motherboard led is on, but that’s about it, nothing else happen.

I was able to get it on once after this behaviour, by just unplugging cables and plugging them back. But no more since.

I’ve tried SMC, NPRAM reset.
When I remove the RAM and turn it on, I’ve the “hardware beep” indicating hardware issue. When I put it back on, its back to the same issue

I take this as a good sign. It doesn’t feel as anything is broken really.

but I’m short of ideas on how to proceed further and try identifying what’s wrong.

Any tips?

Well, as I was about to try T key, the iMac chimed and displayed a white screen that seemed to last forever.

After trying OPTION key and PRAM reset, to no avail, I tried SMC reset by just unplugging the power cord.

After trying PRAM reset again, I just got the linux installed OS to boot, after a quite long time thougfh, like 20-30s. But NO PRAM.

I’m not sure what’s going on but the entire thing is highly unstable. No sure what this is indicative of. Some dying hardware?