I agree with the sound of your CO2 sounding much faster than 1psi.
I had a few struggles with my beer gun when I first got it, but nothing like what you are going through.
Purging the keg is essential IMHO. If it is cold and you purge it gently it will not foam in the keg.
I spray star San in the bottle with a spray bottle, then turn it upside down in a fast rack and put it in the deep freeze. I actually work on a cutting board over the top of my deep freeze right next to my kegerator. I pull out one bottle at a time so they stay cold. Some are frozen by the time I finish.
I also put all the beer line into the kegerator or deep freeze to keep that cold. The beer gun goes in a pitcher down in my freezer between bottles.
When I fill at 1-2 psi it takes significant time to fill a bottle. Maybe 15-20 seconds?
I sometimes get 1/2” to 1” of foam at the top of the bottle when I have things going well. I cap on that foam and seal it up right away.