Desktop Safari: Disable "This webpage was reloaded because it was using significant memory"
defaults read will tell you what options you can set.

It does not appear that there is a defaults write command available as far as I can tell. It's possible to get a list of the commands with defaults read com.apple.safari and there's quite a few that are cryptic, so I could be wrong though (and would love to be corrected on this as I have this problem too with something that uploads and processes data resulting in losses occurring).

There doesn't seem to be an option in the 'Develop' menu to control this. There is an option to 'Get Safari Technology Preview' that might help if this is changed in an experimental version but that's pure speculation.

Safari 11.2.1 on MacOS 10.13.6 - options will be different on newer/older versions.