I used to put my PST+QHY5L-II on a Skytracker and it was ok but I'm sceptical you'll be able to do any kind of long exposure with the ST80 + 700d.
For example I can just get good 30-60 second exposures with my WO71 + A7s on a SW Star Adventurer and, while your scope and camera weigh less than mine, it's a much sturdier mount (and counterbalanced!) and a much larger FOV per pixel.
If it was me I'd probably just look for a nice second hand manual prime lens, which should be a lot better than your zoom. If you can get a Canon FD to EF adapter maybe you can get something like
this. Or if you can get a Nikon ai lens to EF converter, like
this, then you can get something like
this.
I've used similar FD and Ai lenses, with basic adapters, on my A7s and they can be quite good - especially for the money.