I do something similar with my slightly older ETX-90 RA, though I'm more low-tech about it. I'm not really interested in astrophotography, but only fiddle with it because my scope happened to come with a T adapter and "because why not". I borrow my wife's Nikon D3300 and use an ETX T adapter on the rear port. I have a nearly identical PVC dew shield on mine as well. I painted mine flat black and flocked the inside of it, using some extra thick felt pads on the inside so it would fit snugly on the tube and allow me to extend it out a little further. It balances the camera surprisingly well (and is too heavy to use on the scope without the camera attached). The T adapter works really well but does limit how high you can point the scope. The advantage to using the rear port is that you can use an eyepiece as well. It also gives correct (not mirror) images. I have to use an extension (an old barlow lens with the lens removed) to get it to focus while the camera is in focus, but a 26mm eyepiece gives pretty close to the same field of view as the camera. Makes it a lot easier to find what I'm looking for and make sure the image is more or less centered. I just have to remember to flip the mirror down before taking pictures. I have a lot of all black photos because of that. I'm not using any sort of guide scope, so what I'm doing is much more simplistic what what your setup is capable of.
I'm not an expert, but I think your zoom level with a 5x Barlow is probably waaaay more than 25x. Prime focus through the rear port yields images that look (to me) to be about 50x, so you may be trying to take photos at 250x. I've used a 2x Barlow with my camera in the configuration you're using to attempt to take photos of Jupiter and that seemed to be about the best magnification I would be able to manage with the scope and camera. 3x might be possible in good conditions, though. I never could really get the ISO level and exposure length figured out for optimal Jupiter photos, but I only tried on two nights. DSOs are easy: figure out what setting works best and use it all the time. The planets use different settings and the moon is highly... uh... dynamic. Different setting for each night. Haven't gotten the moon figured out yet. Keep a log pf photos and camera settings to help figure out what works and doesn't. If you're having trouble, the first thing I'd do is try unscrewing the lens from your Barlow to get the magnification down to reasonable levels.
My wife's camera can only do 30 second exposures without buying more stuff I don't need, but 30 seconds ends up being about all the unguided ETX-90 will handle before I start getting trailing issues. Having fresh batteries makes a huge difference as well. Once you get it figured out the ETX-90 is a very versatile scope and is capable of some pretty decent pictures. Check out Mike Weasner's Mighty ETX page if you haven't found it yet.