Could you list at least some of the skin barrier site preps you’ve tried? If you don’t tell us what you’ve tried then we are likely to just suggest it again.
Just a complete gut level guess here, but the site prep you describe strikes me as more likely to lead to irritation than to prevent it. It sounds like you are essentially removing any “natural barrier” your skin might have.
Do you run into the same problem no matter which location you insert the sensor at? I find that how my skin reacts seems to depend a lot on which area of skin I use.
Ironically, I seem to be most sensitive on my abdomen. I had a really bad skin reaction when I inserted my Medtronic sensor on the left side of my abdomen. I also have had poor, though not quite as bad, luck elsewhere on my abdomen. But never really much of a problem using my thighs.
With the Dexcom you also might be able to use a spot on your upper arms. At least other people have posted about successfully inserting their Dexcom there.