Thanks for helping to resolve my problem.
I share your conclusions and have indeed attempted changing the baud rate using AT+IPR=9600 on QCom/QNav but it didn’t help.
The Arduino Uno has its limitations when it comes to serial comms but I like using it due to its relative simplicity. Attached some screenshots from https://docs.arduino.cc/learn/built-in-libraries/software-serial
Since this problem emerged a week or so ago I had already ordered and got started with a Nucleo 64 F411RE but as it is a new platform for me it will take a bit longer to set up.
An alternative I will explore is w/ ESP32 & Serial2 but have none in stock at the moment.
Suppose for now I will have to mark this case as closed.
The conclusion for others in this situation is: Arduino not supported, use STM32 instead.