Problem - flashing folder - bootable drive not recognised

Some weeks after replacing the 128Gb card which formed part of a fusion drive and moving from high sierra to mojave, I restarted the 2014 5k Imac but after the screen going black the flashing folder question mark sign appeared. I have removed the memory card / hard drive from the Imac and put in an empty HDD.

If I just turn the Imac on I get a black screen then a flashing folder eventually. If I attach a bootable drive ( createinstallmedia from another IMac ) with High Sierra or Catalina the screen stays black and the question mark symbol never appears.

I have attached a wired keyboard but cannot get any of the standard kypresses to change the situation.

I put back in the original fusion drive components but again without sucess.

I’ve read through the forums which suggest various key presses and only using an equivalent or more modern boot disk but not identifed any solution. The boot disk is being created on a 2010 Imac running El Capitan with downloads of the more modern operating systems.

Several days later I cannot see a way forwards to booting to anything.

Any suggestions?