I would use the same trigger as the other inventory management templates available.
You can either compare the update/created at dates on the product (I think they will be equal if it's the first update). Inventory updates should mark a product as updated. Or possibly you could use tags in some way. Like set a tag on import and clear it using Flow, or set a tag once Flow has processed it. The latter is harder because you'll have to backfill that tag on all products.