iPhone opslag te weinig door foto's voor update

iCloud can not be used to back up your photos, iCloud Photos will only sync your photos across your devices.

You will have to back up your photos from iCloud, if you delete photos from your iPhone while using iCloud Photos, this data will also be removed from iCloud:

Archive or make copies of the information you store in iCloud - Apple Support

In iCloud bewaarde gegevens archiveren of kopiëren - Apple Support (NL)

You can also let the iPhone recommend you steps to free up space on your device:

How to check the storage on your iPhone and iPad - Apple Support

De opslagruimte van je iPhone of iPad controleren - Apple Support (NL)

martijn278, how many GB of storage are your photos taking up in iCloud?

I assume, you have already enabled the option "Optimize iPhone Storage" in the Settings > Apple Account > iCloud > Photos?

It may take a while until the photos and videos will get optimized. But even optimized the photos will need some local storage on your iPhone. Your device will store smaller, optimized versions for browsing, also albums, faces, other metadata for searching. And it will try to keep the most recently used photos downloaded in the full resolution, so you do not need to download them over and over again and waste bandwidth or or have to wait for the download. On my iPhone the optimized versions are taking up roughly 10% to 20% of the size of the Photos Library in iCloud. I would not keep more photos in iCloud than you can afford to keep optimized on your devices.

For example, ich you are willing to use 10GB of storage locally on your iPhone, do not keep more than 100GB of photos and videos in iCloud.

How much storage are your photos taking up on your iPhone compared to the storage in iCloud? Are they already fully optimized?

You can check the iCloud Storage on your iPhone in Settings/Your Name/Apple Account/iCloud ->Storage -> Manage Storage.

Manage your iCloud storage on your Apple device - Apple Support

Je iCloud-opslag beheren op je Apple apparaat - Apple Support (NL)

But, as you mentioned, you can not gain any additional free space for photos on your iPhone, because your photos are already optimized.

If you go to Settings/General/iPhone Storage -> Recommendations, you can see what else is suggested to make more free space available:

How to check the storage on your iPhone and iPad - Apple Support

De opslagruimte van je iPhone of iPad controleren - Apple Support (NL)

Thanks for the helpful additional details, Ingo. and the links in Dutch!