The solution at https://github.com/craftcms/feed-me/issues/368#issuecomment-555710613 works just fine.
(Mentioning this in the docs would be good :)
I didn't bother with step 7 as it seems the first line automatically becomes the default variant if one isn't set.
1. Create a new custom field e.g. "Import ID" and add it to the product type that has ...