I’ve never heard the term evangelical used as a slur, other than by a left-wing atheist.
Even Catholics don’t use it as a slur (at least not the ones I convers with).
I’m not sure where you get the idea the term has a negative connotation, but whatever...
After being a Christian for 55 years I’ll just stay with that term...
I have never heard Evangelical used as a slur either, except by liberals. I have no problem referring to myself as an Evangelical or Born-Again Christian.
When I was Mormon, we used to insult Evangelicals by calling them ‘reborns’ or ‘gracers’.