Hi,
You have a few options depending on how many of the photos you have applied edits to and whether you want to retain the ability to revert to the original photo.
One option is to simply export all 16,000 photos from your iPhoto library (assuming you are still able to run iPhoto) and then import them into your existing Photos library. Photos should identify all duplicates so only the missing photos would be imported. When you export you'll need to decide if you want to export the original photo or the edited photo. If you export the edited photo then you will not have the ability to revert to the original photo.
Another option is to re-convert the iPhotos library into a new Photos library. Then you can use a great program like PowerPhotos from Fat Cat software which will merge the existing and newly created Photos library. Again any duplicates can be excluded. If you are running Catalina then PowerPhotos will now retain all of your edits as well.
It is not possible to directly merge an iPhotos library and a Photos library, so I think the above 2 options are your only choices.