Hello Jim and
to Ice in Space!
You seem pretty certain that a cass/cat is for you and I'm the wrong person to discourage you since I own a couple myself for similar reasons. They may be master of none, jack of all trades or whatever, but a good one will serve you well.
In my experience, the C8 (or likewise Meade 8") will show significantly more and fainter detail than the C6, and is only about 1.5kg heavier, so my sentiment is don't waste your time and money and jump right in at 8"...it's a bit of a sweet spot. Even on the Celestron SE mount (6SE and 8SE use the same mount) the scope is a little under mounted but with a careful hand it is possible to operate without too much vibration and it's a very portable combination. Likewise, there are other mounts that would handle it equally well/better, but those alone would likely eat up your budget without the OTA.
Regarding goto, I'd say it's a must have because the focal length gives you a relatively narrow FOV, unless you have a nice wide 2" eyepiece, so push to and/or manual tracking might be a bit of a challenge.
As for photography...the focal length doesn't lend itself well to DSO photography easily, but this turns around to be an advantage for planetary imaging. You win some you lose some
no scope is perfect...