The standard import scripts are already pretty restart-able already, if it comes across an issue you can pretty much run it again and it will pick up where it left off.
My largest final import output 424.7K posts over 39.6K topics and 10.2K users.
Creating the Digital Ocean instance at 2GB with a 2GB swap and then bumping it to a 16GB instance for the import only required 3 restarts.
I monitored the progress (in a spreadsheet) and when the post imports per second decreased and started to tail off - I just Ctrl+C the import and then started it again.
The import process slows because of the memory hole.
All the while I had at least 25 sidekiq’s running - basically enough to keep the CPU at ~80% clearing the backlog of stuff at the same time as the import.