Hey,
the Atlas EQ and the Guider should be a good combination to do some serious imaging. I think it really depends on how deep you want to dive into the astrophotography hobby and how much quality you demand for your images. I am sure that you can get very satisfactory results with one of the big whites. But the longer you stay in the field of astrophotography, the more obsessed with perfectly round, pinpoint stars you usually get
I also have the 65Q and I am a little suprised that it underperforms the 70-200 with a 2x teleconverter. When i got mine the optics were pinched, which resulted in astigmatism and slightly unsharp stars so you might want to check whether yours is ok in this regard.
The thing is really what kind of objects (whats your favorite? Galaxies, planetary nebulas, large nebulas?) you want to image in the future and how much time you want to invest. Only a few objects are as large as the Andromeda Galaxy, so once you have imaged the brightest objects you will usually want more reach. And as only a few telescopes are corrected to the full frame image circle, they will be costly the more focal length and aperture you want.
But it really depends on your style of astrophotography. If you are the type of guy who spends 5-10 hours on a single object, then a TEC like you mentioned or any other high end refractor (Takahashi, TMB, AstroPhysics etc) will be the thing you want. On the other hand, if you say that your interest in wildlife and astro is really balanced, and you rather shoot multiple objects per night and you don't insist on filling the frame, then you could be really happy with a big white.
Me personally, I really prefer to have a telescope with a dedicated focuser. And if you still go deeper into astro, this would also allow you to switch to a cooled CCD camera if you wish to later on. But if you could buy the Canon lens used, you could still give it a try and sell it later if you develop into this direction.