iMac 27" Boot loop (Everything tried)

I would start by plugging the keyboard into the iMac with its charging cable. This way we can ensure that the commands we are giving the computer are actually getting there. Then, I would try an SMC reset. Without verifying if this helped, go ahead and try a NVRAM reset. This should take care of most issues that could occur with your RAM. After you’ve done both resets, power the machine on and test functionality.

If the issue persists:

Review your system report (Apple Menu > System Settings > General > About > System Report). When the report is generated, go to the left side of the window. Under Hardware, “Memory” is listed towards the bottom clicking here will show you which RAM slots are in use. Depending on how much RAM you have installed, this should show 2 - 4 slots of RAM in use. Ensure the system has properly identified the RAM, how many gigabytes are on each chip, and if the state is listed as ok. If it’s not, it’s a good idea to boot the system with only one RAM chip in one at a time.