Migrating FactoryTalk Historian data to Ignition

That is a lot of data, do they not have compression configured for those points? If they're OK with compression, you might be able to configure it, then reprocess the archives before exporting the data. I'm not 100% sure reprocessing archives would actually run compression on data that's already archived, though. I can probably find out.

I think custom AFSDK is probably a dead end for Factory Talk.