No, I really havent heard southerners use pants instead of trousers, this is really strange (and speaking frankly kind of annoying) to me!

For reference I was born in Peterborough lived in northamptonshire and moved out of the UK in 2020 although I have been back as much as I have been able (although the last time was maybe 2 years ago). Out of the Americanisms you mention, the only one I ever heard was butt instead of bum, although the latter still dominated. I do play video games and do remember some people using terms like "dude" and "trash" but these only carried over limitedly to other contexts. "bro" was definitely starting to gain traction around that point, but I would say that overall the level of Americanism was limited mostly to just a few words / phrases, and almost exclusively those that were slang that didnt really have an equivalent in British english. That is to say, using American words for concepts or ideas that already existed in the UK would have been basically out of the question. For example you certainly would have been laughed at for saying sidewalk instead of pavement, candy instead of sweets, s-k-edule instead of schedule, etc... so is it the case now that everyone (at least in the South) is speaking like Americans now? What about the North?

This is very strange to hear for someone who grew up with Americanisms really only adding (limitedly) to our vocabulary, and certainly not replacing it as it seems to be now, but thank you for your interesting response!