iCloud Photos not syncing due to insufficient storage

You have not bought 50 GB of memory for your iPhone. It is not possible to buy more RAM or more device storage for an iPhone. Whatever amounts of those it has when it is put together at the factory is what it will have for life.

What you have bought is an iCloud+ plan that lets you store up to 50 GB of data on Apple's servers, for backup and synchronization purposes.

What's the difference between iCloud storage and device storage on your iPhone or iPad? - Apple Support

iCloud Photos is a synchronization service, not an archival one. You cannot pick and choose some photos to keep "in iCloud Photos" and some photos to keep on your phone. If you are using iCloud Photos, the system will attempt to keep copies of all of your photos in iCloud, and to keep copies of all of your photos on your phone.

iCloud always gets full-size copies, so if you have 66 GB of photos, and are using iCloud Photos, you will need more than 66 GB of iCloud storage. (Photos probably aren't the only thing that you will be synchronizing or backing up to iCloud.)

As Servant of Cats implies, iCloud Photos uses a very clever method that allows your Photos Library to take up less space on your phone, and yet you can see every picture you have, and you can edit and annotate pictures with little interruption. But to do that, the entire Library must be kept at iCloud.com.

If you want to archive pictures that you don't really want to look at, you can just plug in an external drive, select the pictures, and use Share>Save to Files, selecting the external drive-- and then delete them from Photos, where they will also be removed from iCloud.

Your best bet, though, is to spend $3/month and get enough iCloud storage to keep you from having to think about it at all.

Storage, by the way, is different from "memory." My phone has 8 GB of memory and 128 GB of storage. (Neither can ever be changed.) Memory, also called RAM, is fast, temporary, and expensive. Storage is rather slower, but it's long term and cheaper. An iCloud and external hard drives are "storage," for example.

You should see this:

Understanding iCloud Photos - Apple Community

If you have upgraded iCloud storage and are seeing a message on your device that storage is full then it is possible you are not making the best use of iCloud storage and need to use optimization features. If you do have optimization activated then it is possible you are exceeding your device's needs in terms of on-device storage and you need to buy a device with more storage.

Click on the link to learn how to optimize the storage on a device --> How to check the storage on your iPhone and iPad - Apple Support

If your storage is full, use recommendations to optimize storage

Your device might offer recommendations for optimizing storage in the Storage section of Settings:

Tap Show All to see the recommendations for your device.

Read the description of each recommendation, then tap Enable to turn it on or tap the recommendation to review the contents that you can delete.

Optimization specifically with reference to "Set up and use iCloud Photos" - Set up and use iCloud Photos - Apple Support

--> Save space on your device

iCloud Photos keeps all your photos and videos in their original, high-resolution version. You can save space on your device when you turn on Optimize Storage.