Your current lens, scope, is clearly producing tight color-free stars. You have what you need now.
It really depends on what you want to image. If you're after much smaller, fainter objects, when sure, there's room to grow. But imaging at 900mm focal length will require significant mounting requirements and a smaller sensor camera will exacerbate this.
I would not get an 80mm APO at this time. You clearly already have a good "refractor." Your camera lens is one and it's well corrected at this scale at least.
You probably need to take a moment and really look into what you're looking to gain or move into. If your goal is to stay with a dSLR and keep after DSO by getting a new scope, I would say... this is not the way to go. The way to go is to use this current lens/glass you already have, its clearly good, and look into a better sensor camera, like a cooled color camera or cooled monochrome camera with an HA filter at the least and start working with this. You do not need a big fancy aperture refractor to do this. You already have a good "refractor" in your lens. You will do far better having a better camera to handle noise and narrowband to handle light pollution. And a mount is more important anyways to even allow for long exposures in the first place regardless of what you do.
After you get a new camera, new mount, etc, and have it all under you belt and you're producing good long exposures and collecting hours of data to do integration time, then consider a Quad APO like this.
DSO imaging requires:
1 - Time first (doesn't matter what you're doing if you don't have time to devote to integration)
2 - Mount second (can't do long exposures for hours and hours if your mount can't do it)
3 - Cooled sensor (insert narrowband & HA filter minimum if in light pollution here) (because signal to noise and calibration makes the difference)
4 - OTA (the least of your worries)
5 - Lots of processing experience/time (this is really closer to #1 but cannot be unless you have data in the first place, so it by logic must be last, but it's critical)