iCloud sync issues

I’ve just bought a new iPad Pro and soon to have a new iPhone too. I’ve never really used iCloud before but now I want to get all my photos onto the iCloud so I can view them from my new devices. I currently have photos stored on my current iPhone and my MacBook Air. I want all of these to be accessible from the iPad’s photo app. I thought I had set up everything properly, all software is up to date and the obvious photos selection on the iCloud preferences is turned on. However, my phone has only pushed through the last 5 photos i’ve taken. These are showing on my iPad now, but no others. I have over 49GB of free space on the cloud so i’m Not quite sure what the issue is. I have 4000 photo on my phone (I’ve had it 5 years) and I don’t want to lose them.

Any suggestions?