canon - How to shoot nice shots in indoor nightclub?

Using an EOS80D with an 18-35 2.8 Sigma lens I recently shot at a nightclub and had done some research first. I wanted to create shutter drag with my flash and shutter speed. That kind of worked, but didn't really look nice IMO.

I then wanted to take some regular pictures, but I noticed that my backgrounds were pretty dark. I thought I could increase the intensity of my flash using the settings of my camera, but that didn't really seem to affect anything... I just kept thinking that the photos were bad. I went to the owner to tell him that I was screwing up and already apologized for the bad outcome. He told me to just keep trying and so I did some live research during the party about suggested camera settings. I feel like I already really know well how to set my camera for regular daylight in terms of shutter speed, aperture and iso, no problems there. But nightlife? I couldn't figure out how things were connected.

I read that my iso shouldn't have been at 100 (around 400 rather), but I don't really get that. I would've thought iso 100 would be perfect since I'm using a flash and the room should be lit because of that flash? I noticed that I couldn't pull off any overview pictures that way though (so from the back of the room still being able to see the dj). Increasing ISO seemed to work out.

I read that my aperture was also way too high. It was at around 9 while I read that it should be around 3.5-5.6. I thought, there will be a lot of light from my flash; I can crank up that aperture and then I won't have any out-of-focus images at all. However, lowering my aperture did seem to make my pictures somewhat nicer.

Also, I couldn't really tell what my shutter speed had to be. I knew, for the shutter drag, I needed a longer one (1-2 seconds) but I read 1/15 - 1/30 for the nightclub photos. I can't really tell why that's a good number. My photos started to get a little better, but I also had a hard time aiming at people because I couldn't look through the viewfinder. I often noticed afterwards: "Damn, I missed that moment because I didn't aim well... I wish I had a way to still see what I'm shooting". I tried setting the view on the flip screen, but then my camera wouldn't flash anymore for some reason.

This is one of the most fun pictures I got to shoot that night and I still don't like the way it looks. Is everything from this point on lightroom/photoshop post production to make it look nice? Or do I still have a lot to gain by getting my hardware right? I would really like to also understand the theory about the settings behind it so I can easily adapt to different indoor lighting situations. I also used a very old flash (chinon 277) and I'm wondering if that's also a problem. Do I need a different flasher which can tilt? Do I need to use that little card which I've seen other people use to soften the flash? Because I also felt like at some point my flash started to annoy some people, and I can definitely understand.

This kind of was a horrifying feeling for me because I was not in control at all. I don't want to deliver work like this and would like to get to know every little detail that I'm not grasping / don't know about.

I went to look at previous pictures for that venue and noticed that there's a clear difference, I just don't know how to get there exactly. I notice there's been some editing going on, but still, I would like to be able to get to this random example: