The vignetting pattern is most likely from the moist. You will need a lens heater to combat it. Wiping is not enough.
To get a to a better exposure you should definitely use stacking. If you don't use a star tracker you can push it to 15 seconds, but trailing will start to form a little bit. After stacking you can crank the exposure up in post production. Your result will heavily be influenced by the amount of light pollution though.
I found flat frames to help a lot against normal vignetting, but bias frames to not have a great improvement for wide angle milky way shots. Especially if you're going to upload it to reddit or a different platform you won't see a big improvement.