speaking as a local , you could have gone round by macclesfield and over the cat and fiddle just as quickly and with a lot less wear and tear on the truck . that a54 is not fit for loaded top weight trucks in fact most firms in this area would sack you for using it unless empty .
Perhaps use manual a bit more OP. I know you’re new and I’m not criticising you but you can’t expect an auto in a truck to behave as a automatic car does.
You have to interfere with an auto much more unless it’s iShift from Volvo (also in Renualts) in which case there’s much less need to interfere but you still have to occasionally.
It sounds to me like you’ve been thrown in the deep end with little or no job specific training. Might be worth going back to a training school and having a lesson on a freighted motor. I guess you passed empty