At the stage where it’s at now it doesn’t pick up where it left off, because it’s currently running my home-grown private message import function. I did try extensively to code it so that it would resume from the last batch number like the import_topics and import_replies functions do, but something about the algorithm in my private message function is preventing it from doing that.
I realized that Discourse had taken an automatic backup about 36 hours into the import process and apparently nothing bad happened, so I went ahead and ran another backup and exported it to another VPS and restored it there. Seems to work fine, and the importer is still going strong.