with this many pictures it is likely you have icloud photos enabled and many of the photos may not be stored on your iphone 7 plus. If this is the case and many of the photos are on icloud you may have photos with the same name hence the (1) being added to the file name. to check this go to iOS settings > Photos > and see if iCloud photos is enabled. if it is photos can be stored locally on your iphone 7 plus, on iCloud, or on other iOS devices you may have or have had. to see what is actually stored locally to your iPhone 7 plus turn off iCloud photos and give it time to desync. Then check the native photos app and see haw many pictures are actually stored locally in the camera roll.
the ixpand drive app does not convert any picture. If it was shot in jpg it will transfer a jpg. if the photo was shot in heic it will transfer heic. No conversion takes place in the ixpand app. the mix would either come due to the photo being taken before iOS 11 which added heic or the setting was changed from default heic to jpeg.
