Stop iCloud filling iPhone

- I have an iPhone with updated iOS

- I have a large photo library

- I have an iCloud account with over 100gb of free space

- I have turned optimised storage on

- So why is my iPhone constantly telling me it's out of storage space and that I cannot take any more photos/ videos?

Arggggggh!

Either I'm doing something drastically wrong, or photos / iCloud is fundamentally flawed.

Can anyone help?

I think I understand how iCloud is supposed to work. It stores full resolution photos in the cloud and uploads only the (full res) photos you have space available for to your phone/ device... right? The rest are thumbnails, until you free up space.

I suspect my thumbnail library is so big that it's causing problems, but is there any way to see if this is the real problem?

Or do I have too many apps? - I tried deleting, but I can free up 700mb of space and it gets clogged again in no time with new uploaded photos.

Is it possible to 'limit' the amount of free space left on your mobile device (I suspect not)

I'm an apple fanboy of 15-20 years and this issue (combined with a few more I've uncovered recently) is seriously tempting me to jump ship.

Surprisingly I can't find any solution online.

I feel like I'm paying for a service which simply does not work at the moment