My house has a built-on porch, the original front door was left in place
and a new front door installed, so the porch was a separate room, I
have removed the original front door, so the porch is now effectively
part of the hall.

Having removed the original door, frame and step, there is a difference
in levels between the concrete floors of about 40mm, the porch being
lower, the DPC is only about 25mm above the level of the porch floor, so
adding 40mm of concrete seems like a bad idea, is there any thin/strong
enough insulating foam blocks to make up the level?

I suppose I could either frame around the porch floor with about 1"
timber, with a few cross-members to subdivide the span, then fill the
spaces with foam, and then cover with plywood.

Alternatively I could forget the frame and just lay about an inch of
foam block with plywood over that, how much will the foam "give" over time?

Or, I could just say sod the DPC and fill with cement/concrete, any
other thoughts?