Your raw data on Bode's Galaxy looks good. The image labeled "without post processing" is impressive. There is quite a bit detail in your raw stack. When you process the data, try to preserve as of the detail as you can. One way to go about this is to use masks to protect the areas of the image you don't want to be affected. This can come in handy when you are using curves, for example.
Another thing you might want to consider is to enhance the dynamic range of the galaxy around the core of the galaxy to reveal more of the subtle detail. Right now, it looks like a lot of detail is lost or washed out with the light from the core of the galaxy. I am not sure how to fiddle with the dynamic range in Gimp and don't want to advise you incorrectly. Perhaps someone else may be helpful in that regard. In addition, there is also an interesting nebulous region just to the right of Bode's galaxy which is hard to see in the processed images. It also looked like the stacking process didn't remove all of the satellites, which is not uncommon. You can try and re-stacking again using a different rejection algorithm in SIRIL or use a different piece of software to stack the data altogether, for example Deep Sky Stacker. http://deepskystacker.free.fr/english/index.html
I hope I don't sound too critical, your data is indeed excellent. All of these small little fine tuning adjustments in post processing are all just personal preference. A lot of it depends on how much time you want to put into this image to learn your way around the software. Having good data to start with is definitely a bonus when learning your way around astrophotography processing software. Keep up it, what ever you got going on now is headed in the right direction.
I have never used SiriL but I just downloaded and installed it. At first glance, this definitely looks like it's a useful piece of software. I still need to learn my way around but I think it's worth keeping/using. However, as far as gimp is concerned, my best advice, is to use another piece of software for processing astrophotography data. I don't believe there are many people in the club that use Gimp exclusively for pre/post-processing. It's not that easy to use and it might be hard finding someone to help.
PixInsight is my go to software for processing. I also find Adobe Lightroom handy as well. I use the mobile version of Lightroom to make final adjustment on images intended to be viewed on a mobile phone before I post to a website like FB or Reddit etc...
There are tutorials all over YouTube that demonstrate how to use PixInsight for various astrophotography post processing techniques -- aside from hundreds of YouTube Videos available, many of us in the DVAA use PixInsight for astrophotography. PixInsight is the software I would recommend above others especially for those just starting out with this hobby.
Adobe Photoshop is also very useful for post-processing astrophotography data however, I would only consider using it if I already had access to it for free. I would not run out and buy it to learn how to use from scratch. I am personally not very familiar with Photoshop but I think we do have or 2 two people comfortable using it for astrophotography if you already happen to have access to it.
Canon EOS Rebel XS (full spectrum mod)
Astronomik CLS CCD
Orion thin off axis guider
QHY5L-II M guide camera
Celestron CGX,Edge HD 8,0.7x focal reducer
Optec TCF-Leo focuser There is also an interesting nebulous region just to the right of Bode's galaxy which is hard to see in the processed images.