MAcbook COMPLETELY STUCK; disk is full, can't transfer pictures, can't update the system

Hello!

My MacBook is completely stuck. I cannot update the system, since there's is not enough space, yet i can't create more space by transferring pictures into an external hard space. I'm currently running by Catalina 10.15.7 i know i should've update some while ago...

Here are the problems i ran into;

I cannot transfer my pictures from photo library into an external hard drive, some of them, because there isn't enough space to download the full content of the video or the picture. I cannot watch some old videos for this reason for example. These kind of pictures are immovable from the Photos. And the most space taking media are videos indeed.

If i copy paste, some of them can be copy-pasted and some of them can't.

And I also bought i huge amount of iCloud space, in hope that space would never ever end again, yet, now im a bit confused how i am supposed to transfer pictures into iCloud and not store them in the Photos at all, since i have a lot of photos.

The ideal would be having the whole photo library in the iCloud or in the external hard drive, but now it seems that im unable to do neither. I have done all the optimization and still full.

I guess im not realizing how the iCloud fully works or is supposed to create more space for the devices?

Can anyone help? Taking in all the possible possibilities of solutions! Thank you so much in advance!