Home DIY

You'll need to fix the hardboard. It might move around otherwise. You could pin it to the floorboards or staple.

I'm a big fan of cork, nearly went for it with my workshop but went for a very heavy lino glued down in the end. The workshop floor takes a lot of punishment though.

Have you considered trying to feather out a screed to lose the height difference between the two surfaces?

Do you need to feather/taper? Will be much easier without but might not work with your door at the other end of the room.