Two iPhoto Library in one... How to combine

Allow me to ask you another question as you are solving so many problems and thank you in advance.

My situation is:

- I currently have the synchronized photos of my Iphone 7 on iCloud from 2017 to today 2020. I also have these photos on my Mac in a Photos Library, on the internal hard drive, as it has downloaded them from iCloud precisely.

- The second Photos Library of my old iPhone 6 of 2015-2017 photos is on an external hard drive.

I'm following the instructions Notes on Merging Photos Libraries, 2019 V… - Apple Community and I understand that I have two alternatives:

1. Merge the libraries in iCloud by uploading them to the same iCloud Photo Library

2. Merge the libraries by exporting the photos

The best of the two is the first solution. Also because I saw that uploading them by hand to iCloud with the second solution, allows me to upload only photos and not videos.

I state that I have activated the 200Gb plan on iCloud and that I have enough space for new photos.

So I'm trying to follow the instructions of the first solution, but I don't start uploading photos to iCloud and I don't understand why. I'm doing:

1) Open the old library (2015-2017) which is on the external hard disk and set it as the system photo library

2) activate iCloud in the Photos app

3) clicking on iCloud photo, it gives me this message: Only items copied to the library will be uploaded to "iCloud Photos".

4) I leave these settings: Optimize Mac storage

Now on the iCloud site I should see the photos from this library added to those that are already present in the cloud. Quite right? What do I have to do?

Sorry for the length of the text but I wanted to be as clear as possible.

They are two different libraries. One related to my old iPhone 6 which is now saved on the External Hard disk with old photos and videos up to 2017 (which I don't have in the one I'm using now).

The library I'm using now, however, is related to the iPhone 7, from 2017 onwards. What I asked for was to be able to join them.

ah ok now it's loading me. I solved it by copying the library to the internal hard disk. Maybe he had some problems because the library is on the same hard drive as the mac backup.

Now then, once I have finished uploading the classic library back to the mac, I should be done.

Yes, the second, Photos Library