Hi Roger,
I agree with both what Greg and Lee have to say.
When I was using an C11 + ST8 combo at 0.5"/pix I found 10 minutes subs to be a minimum. If the sky was dark enough (and I suspect they are for you) 20 or 30 min subs would be even better, especially for RGB where the signal is at least a third of lum. I still do 30 min subs with the CDK20 at f/6.8.
There is a bit of collimation issue, the bottom is better than the top, but not much. What you are seeing here is the inherently small corrected field of an SCT. So you are seeing coma. You question your optics, but that is easy to check, under very good seeing take short (~1-10 sec) exposures and measure the FWHM. If it is < 1.5" then the optics are probably good enough for your site, you are seeing limited not diffraction limited. The detail visible in the inner bar and nucleus would lead me to conclude the optics are fine.
The background is slightly black clipped as Lee mentioned, and you are stretching the data pretty hard. The bright stars could be handled a bit better, but again using an SCT I always found it difficult to keep the really bright stars looking natural. I find the diffraction spikes in the RC/CDKs help distract the eye enough from the funny looking disk. The little disk in the core of bright stars is a sharpening artefact, you can use a mask on the bright stars when sharpening other details to prevent this.
Here's a link to a similar brightness/size Northern hemisphere galaxy taken with my C11 and ST8 on a G11 in my suburban Ottawa backyard back in 2006.
https://www.faintgalaxy.com/ngc4651.html
It also had 'optical jets'. Notice the stars are a bit rounder near the edges of the chip, but I had the Celestron reducer-corrector, I don't think the CCDT67 does any correction, it's only a reducer.
N.B. NGC1097 is actually a tough object to process, the range in brightness between the core and faint extensions is huge so you have done really well here.
Cheers,
EB