Stoker Insert Pipe Height

I am inserting a stoker at the front of my masonry fireplace and venting up the flue. There is an area of about maybe 3 feet above the damper that has staggered brick that has up to 1/4" buildup of wood soot in places mainly corners and then goes up to a very clean clay lined flue. I was thinking of connecting a 6" pipe to the burner vent, going through the damper area, and ending in the clean lined portion. Is this correct process and place to end the pipe ?

Also, I am thinking I need to place insulated covering to close any gaps between the stoker and the fireplace opening and will have a mantal shield since the wood mantel is about 12" above the stove top. Is there anything else for install or something that I am missing?

Clean it or pay someone to clean it.
Better to be safe than sorry.

I don't have a problem cleaning it, but once clean can I vent into it then? It doesn't appear to be lined at all. All I see is staggered brick and mortar

The proper way would be to run a flexible stainless steel insulated liner to the top and install a cap. Use a "T" at the bottom to allow for fly ash cleanout and a barometric damper. Then you are safe and efficient. That unlined fireplace chimney is most likely way to o large for your stove to vent well.

I eyeballed I think an 8" flue size but I will measure to be sure. If the flue is sized properly (I do understand 6" is what I want), in terms of collecting ash would I connect the stove pipe to basically an upside down hood sealed at the flue to collect ash? I also thought about running a vertical pipe from the elbow to the flue and insulating around the pipe past the damper without a hood but wonder if fly ash will exit the stove pipe and collect on top of the insulation.

What have folks done and found in terms of the amount of fly ash exiting a hearth stove pipe?