I was unaware that there is a difference in RAM based on Storage capacity with the 2018 iPad Pro 3rd Generation models.
I sounds like you chose a Storage capacity of less than 512 Gb?
All the ‘experts’ were quick to say the 2018 iPad Pro is the one to go for but nobody, including the Apple Sales person I dealt with, mentioned the ‘small print’. So small infact that it is missing from current Apple iPad blurb on their web site.
Understanding your predicament it is appreciated that you may be reluctant to embark upon a series of ‘experiments’ to find out what your iPad can handle.
You may find this helpful:
https://discourse.shapr3d.com/t/faster-way-to-copy-things/5381
If the File is sensitive then there is nil option but to ‘begin’ the task, often one started thing get easier. Although it does not sound like a five minute job.
Given the size of your File if you are prepared to place it on a Download Site, advising info via PM of course, I am happy to investigate possibilities.