Okay, a couple of things.
1. How is your polar alignment? You have to have a good polar alignment to have good tracking. What kind of guide scope and guide camera are you using? Here's another question. What software are you using? For Astrophotography, I would be looking at N.I.N.A. since it's free and pretty powerful. It also has a built-in polar alignment feature which will help you get polar aligned.
2. How is your actual mount in terms of movement? When I got my AVX a few years ago, I could put all sorts of stuff on it and it would be horribly imbalanced and still wouldn't rotate because of the imbalance. I followed this gentleman's advice on the rebuild of my AVX. When I opened mine up, it had axle-bearing grease, the really sticky sort of stuff, and I ended up replacing it with white lithium grease, and when I did that, it was very smooth, and would easily rotate around it's axis if it was out of balance. Before that, it wouldn't rotate at all.
https://rocketsparrow.blogspot.com/2017/01/how-i-made-better-celestron-avx.htmlDon't worry about using the bearing, you don't really need it. I don't have it in mine. The rebuild will take you a couple of hours, but it's really not that hard.
What are your guiding numbers looking like? Can you post a PHD2 guiding graph (screen capture?)
Others have mentioned changing the gear lash. I've found that if you do a deep 'clean' on the mount and switch to lithium grease instead of the axle-bearing grease they used (at least in mine) you'll not have to worry about it since the mount will move far more freely.
Okay, I have several suggestions for you.
First, a Schmidt-Cassegrain is the absolute last telescope that I would want to use for start out in astroimaging. Have you downloaded the free Stellarium application? It's a software based planetarium. You can simulate several telescopes that you put in, as well as several cameras, and a whole bunch of other stuff. My recommendation to you is to install the software on your computer, and then go into the settings, put in the information for your SC 8 and then the camera below. Then, look for M31, the Andromeda Galaxy. Switch between telescopes and look at the image that you'll see with both. There is no doubt that the SC8 has it's place, but not for a beginner. Your profile doesn't say where in the world you are, I'm going to give you links to U.S. retailers, you can find something similar in your country or region if you're not in the U.S.
Astrophotography is not inexpensive. The stuff I'm going to recommend is not going to be cheap.
Telescope. MY RECOMMENDATION is to start with a small refractor.
From High Point Scientific, the Askar 71 Flat Field Refractor is $600. The nice thing about this telescope is that you don't have to worry about 'back focus' and a bunch of other stuff.
https://www.highpointscientific.com/askar-71f-flat-field-refractorYou can add the Bright Focus Mask and the 32mm Guide Scope, and I would recommend both
Guide Scope:
https://www.highpointscientific.com/askar-32mm-guide-scope-black-32gs-bl Add $97.
Focus Mask:
https://www.highpointscientific.com/apertura-bright-focus-mask-for-60mm-to-90mm-telescopes $25. If you're unfamiliar with this, look up 'Bahtinov Mask' to learn what it does, but it's for focusing accurately.
Next, I see you have a StarSense GPS. That's great if you want to know where you are in the world, but it doesn't do anything to align your telescope. There is also the StarSense AutoAlign, which will figure out where you telescope is pointing and makes for great 'Go-Tos' but does not polar align your telescope.
https://www.celestron.com/products/starsense-autoalign You don't need this is you're using a laptop.
For really good polar alignment, since you're using an AVX, I would recommend using the QHY Polemaster. This device, in addition to a laptop, will allow you to precisely polar align a telescope. You'll also get an adapter, which must be specified, to fit the AVX. You plug into the computer and run the alignment software and you'll be set for a night of imaging. The adapter is another $30. So, $300 total.
You'll notice I keep coming back to Polar Alignment. There's a reason for that. Your accurate, precise polar alignment is the rock bottom foundation of good astrophotography. If you don't have good polar alignment, you can have the best equipment in the world and will get results that are horrible, or even total failures.
Okay, so now we have a new telescope, a new guide scope, a new camera for polar alignment, let's get a camera for the guide scope.
I think ZWO makes great stuff, and the ASI120MM should work well for the guide scope we're using.
Look up the ZWO ASI1200MM guide camera, about $130.
You'll need software to run the guiding. PHD2 is pretty much the standard. My hyperlinks aren't working right, so look for 'openphdguiding.org' to download it.
You might want to get a star diagonal and a couple of eyepieces so you can view what you're looking at.
Okay, so you have your mount. Rebuild it according to the instructions above.
Then, you'll get a new, smaller, wide field refractor telescope.
You'll need a real guide scope and guide camera.
You'll need software, NINA is free and so is PHD2 Guiding.
You'll need a laptop to run everything.
For your camera, check out 'BackyardEOS. You can get a trial version, a Classic Version and a Premium version. Since I don't have a Canon camera, I don't have any experience with it, but it's always been well spoken of. I don't know if you can integrate it with NINA or not. You may be able to use NINA with the camera without BackyardEOS. Maybe others can comment on that.
Based on what I have seen and read from you so far, you are missing some stuff for serious astrophotography. This list will help to fix some of that.
Finally, I went through some financial difficulties and lost a good portion of my astrophotography gear. I still haven't had the money to get back into the hobby, so I am not actively in the hobby right now. I wanted to put that out there. But, the information I've given you is still good
I wish you good luck and clear skies.
Gary
min subs are doable with decent hfr.