I have two goals.
One is to minimize creosote in a 30 ft+ exterior chimney. So I think a standard insulated flue would be fine.
Two is to minimize the heat loss up the chimney. Obviously a block off plate and some rock-wool just above the plate.
Is the thought that a pour-in of some type would accomplish both goals? And cost less than putting a new liner in or pulling and wrapping the current liner?
No reason to over think this.
Block off plate at bottom with rockwool above it (will keep the pour in stuff at bay)
Get a few bags of the pour in insulation and dump it around the flue.
Top plate.
Enjoy warm house.