Have you tried imposm? Your process sounds magnitudes too slow.
Re:OpenStreetMap Is in Trouble
by 6487698hhhh on Saturday February 17, 2018 @10:13AM
(#16401003)
Agreed. I can load with osm2pgsql on my home PC (AMD FX 8370, 32gb ram, 1tb SSD) in a couple of days, and that's generating 78 output layers. imposm is supposed to be faster still.
We need servers with at least 24 CPU threads and 256 GB RAM and moving the data into PostGIS, region by region takes a week or two. Plus you manually have to fix the process when it just spits out an error and aborts after 15 hours of churning.