Is it safe to eject the external storage at this point?
The whole point behind safe to eject is macOS mounts the drives with the write cache enabled. That means it may cache some of the data while it writes the rest. If you eject before the cache is written out, you could lose that data. On a spinning hard drive you could possibly damage the platter, but probably not.
It would be rare that it would still be writing cached data more than a few minutes after it looks like it has finished writing in the Finder. The unsafe eject message doesn't mean it was still writing, just that it could have been. It has no idea--if it could know, it wouldn't eject the disk until it was ready.
So, by now, it should be safe to eject, even if you just pulled the USB cable and didn't unmount it.