How to transfer pictures/images from one Lightroom account to another?

My advice would be to ignore the Downloader option, there's just too much stuff that it doesn't download. Instead, assuming both accounts are still currently accessible, you could do the following:

1. Login to your new account, open Lightroom and create a new empty album and set it to be shared. Use the "Invite Only" option under "Link Access", then enter the email address that's related to the old account, use the drop-down list under the "eye" icon and choose "Can Contribute" then press the Invite button.

2. Then logout of Creative Cloud and log back in using the old account details. Find the email that you've just sent to yourself and click on the link. That will open the shared album in a browser window, which is how the authorisation is accepted. Now open the Lightroom desktop app (still using the old account credentials), click on the "Shared" tab next to the "Albums" tab below All Photos, then click on the "Shared with You" tab. You should see the newly shared album in the list, if you do click on All Photos to populate the grid with all of the 7000 images in the library. 

3. Depending on your operating system use Cmd (Mac) or Ctrl (Win) + A to select all images, then either drag them all (by dragging from the centre of any one of the selected images), and drop them on to the new shared album in the Shared Album list. Or right-click on any one of the selected images and choose "Add photos to Album", using the flyout list to select the new shared album.

4. When you add the images to that shared album you'll see a dialog box asking you to select what attributes you want to include, so make sure you check them all. Then wait until Lightroom finishes syncing (i.e. the blue spinning circle disappears from the Cloud icon top right), after which you can close Lightroom and logout of Creative Cloud.

5. Log back into Creative Cloud using the new account details, launch Lightroom and you should see that the shared album now contains all of the 7000 images that you "contributed" from the old account, and you are now free to organise them as you wish. CARE: there's a 48-hour window for the contributor to change their mind and delete those images from the shared album, so don't do anything drastic such as closing down the old account until that 48-hour window has passed.