How do I save my photos from an old MacBook to iCloud?

See this article https://support.apple.com/en-in/guide/photos/back-up-the-photos-library-pht6d60d10f/4.0/mac/10.14

• Manually copy your library to an external storage device: Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy. 

If you have more than one photo library, be sure to back them all up.

See this also https://support.apple.com/en-in/guide/photos/create-additional-libraries-pht6d60b524/mac

Note : the external storage device must have enough storage space to accumulate the iCloud photos .

Hello Lilshe and welcome to Apple Support Communities.

I see you're having some issues getting photos from a pair of older MacBooks. I've run into similar issues with first iPhoto and later the Photos app and older devices, so I very much understand what's at stake here.

As a first step, we'll need to make sure that you have enough iCloud space. A free iCloud account includes 5 GB of space which is not going to anywhere close to enough for more than 11K images. If you're using a 5GB account, start here for info on increasing that storage space.

Buy more iCloud storage - Apple Support

Cheers.

Hi Lilshe,

In your original post, you said that after uploading some of the photos, the system appears to become "overwhelmed." Are you getting an error message that says the process has stopped? With that number of photos, uploading will take a very long time. On the order of days, not hours. And this will be 100% about the speed and robustness of your Internet connection. Is the Mac going to sleep during the process? If it is, that could be an issue. You can set the Mac to not sleep on a temporary basis at System Preferences > Energy Saver. Just move the slider there all the way to the right to the Never setting. You'll want to put that back to wherever you normally keep it set after this process has completed.

Cheers.