crumbling chimney/conversion to metal pipe

i asked about new flashing/rebuilding chimney top last night, but as i am looking more closely i am finding that my unlined chimney is pretty crumbly and unstable, and i am hesitant to rebuild with brick as it will most likely just fall apart again. i am planning on running a stainless liner anyway, but im now thinking it might be better to hook the liner up to insulated pipe and run pipe through the roof (either from the roofline up or even from the attic floor up). the chimney is centered as it comes through the ridge. does someone make a one piece ridge flashing/pipe termination/cap assembly that could be placed over the whole mess that i could hook the liner up to? and would ice damming be an issue within the space under this flashing? i dont know if this is out of anyone's realm of expertise, but any info would be great. picture is attached. thanks so much