Any reason not to use oval (6"-equiv) rigid liner, offset permitting?

This site is a total trip...addictive too, trying to learn what I can the past couple months.
Thx all for being here!

Finally ready with a question(s) of my own:

With a flue like the following, does a rigid SS oval liner make good sense? Can it reasonably be insulated with something other than pour? (Do rigid liners have 316ti on the inside?)

· ceramic flue w/ inside measurement of 6.7” x 10.7” (or so)
· 19’ from damper to top of flue.
· top of 3-flue chimney is outside the house for 7’ on one side and 12’ on other
· flue for this fireplace is in center. One other flue is exhaust for house’s primary heat—gas furnace (and was warm today!)
· There are “concrete snots” [I learned that here!], but not much “offset”
· 26-yr veteran fireman-now-chimney-repairman& sweep assures that flue is fine for burning
· NFI-certified potential installers are willing to knock out some masonry, so damper size is not an issue.
Goal: install an Osburn 2200i w/ 6”-equivalent (rigid or smooth inside dual-ply flex?) liner with *some* insulation
Pix attached! (video TOO large!)

Yes, if you oval it you can use wrap insulation.

No, they are 316L or 304L. 304L is for wood only, anything else requires 316.

Obviously depending if rigid can fit, depending on the offsets, why not use round rigid and 1/4" wrap insulation?