As far as I can see in the slack documentation, you can export dateranges and this is what I did back then when I migrated from Hipchat, because the single file was too large and I wanted to test the delta imports anyways, so what I did was export year 2017, import it and see if it works, then exported year 2018, import it additionally and continued until I reached a pretty current date and then I started to make daily exports and the final export was happening at 00:01 for the day before (which only took a few minutes then, export, download, convert, import) because there wasn’t much data to import anymore and the method was tested a few times already for the imports from the years before.
With the Hipchat export back then, you could choose to exports users and so I did only do that for the first export, not sure how this will look like on slack, but just give it a try and let me know (not using slack here so I do not have any experience with that, sorry), but in the worst case, you will need to skip the users from all further imports and just import the channels.