You can't use formatting string parameter in an expression as it works like a Nanoflow. It will use the decimal format from your locale.
https://docs.mendix.com/refguide/parse-and-format-decimal-function-calls/
One solution could be to create a new attribute in your entity and set it in your microflow. Make it one of the last actions of the microflow, so it uses the values that have already been calculated and stored earlier. If it's using the calculated values, you won't need remember to also change it in the "total" calculation.
Good luck
Many thanks Robert - I had thought of this too (using multiple additional items in the microflow - it works) but I had hoped there was a way Mendix could control the output on the client side. Really appreciate your help - all the best