I don’t have an 8mm rod to test, The 8mm firmware I was sure took care of it. Try slowly increasing the feedrate. I’m sure you can go faster than 50%. No one else has had a problem with the 8mm so maybe your machine is just tight and needs to break in.
You can use cura, no problem.
When you get the nozzle at the build plate, or 0,0,0, make sure the position reads 0,0,0. if not reset the arduino or hit set home offsets on the LCD.