Coaxial warp of course, shares numerous stability problems with slipstream as a form of transwarp travel, with the notable exception that Coaxial Warp would be a thousand times worse than any subspace weapon if it went unstable. In Vis a' Vis, Steth's ship would have collapsed
space within a radius of about a billion or so kilometers if it's coax engines had gone kaboom.
If I may be so bold, I would suggest that you not consider any exotic transwarp propulsion systems, since most of them are unstable and poorly understood by Federation science, and instead go the route that I proposed the Excelsior had gone - that is, to turn conventional warp drive up to 11.
Or, perhaps, even introduce a new understanding of warp propulsion to Federation science that allows ships to take advantage of natural phenomenon to achieve transwarp speeds when needed (like the increasingly-popular fan theory of "warp highways"). Such an approach would probably be for the better if the sim is set only a few years after DS9/VOY.