Morning pal,just had a look on your wheel size you are about to change to,but that's not good idea mate,if orig.tire size on your allroad is 255-45-19 and you'll change to 235-55-18 your speedometer will too low compare to actual speed,60mph on speedometer actual speed will be 60.4,no good for speed cameras mate,it's just opinion,and for the offset have a look here,this might help
https://www.wheel-size.com/size/audi/a6/2018/#trim-20tdi-eudm-188