Things to Consider before using Optimise Storage on the iPhone or iPad for Photos

Some things to consider when going for an "optimised" iCloud Photos Library on your iPhone or iPad (Set up and use iCloud Photos - Apple Support):

It will depend on your amount of storage on the device, your internet connection, your patience if you will enjoy it. And it may not be feasible for you, if you frequently away from the internet.

• The big advantage: You will save storage on your device, because the high resolution originals will be stored safely in iCloud and replaced by smaller versions on the device. If you turn on Optimise Storage, iCloud Photos automatically manages the size of your Photos Library on your device: Manage your photo and video storage - Apple Support.
• The disadvantage: Accessing the photos will require a download from iCloud - this can be slow or not possible at all. And backing up the photos will be more complicated than before.

Because some photos will be optimised, you will only be able to use them if you are having a reliable and fast internet connection, when you want to use them and Apple's cloud servers are not down. It can be hard when commuting or on long distance flights, or in any area with a poor internet connection or abroad, when internet access is expensive.

Because we cannot control, which photos will remain stored locally or get optimised, you may find yourself occasionally unable to access a photo that you want to use. This may be embarrassing, if it are business documents, that you urgently need, but cannot access.

You have to plan ahead, how large you want to let the iCloud Photos Library grow. The local storage on your iPhone will be the bottleneck, determining how many photos you can afford to have in iCloud. On my iPhone with the smallest storage the optimised versions are having a size of 10% to 20% of the size of the library in iCloud. So, if you can afford to have 20GB of optimised photos on your iPhone, I would not let the iCloud Library grow much more than 200GB, or Photos will not even be able to create optimised versions on your device and you will only be seeing blank thumbnails.

You should also plan ahead, how you want to backup your photos, once you are using "Optimise Storage". The iCloud Backup of the device does not include the photos anyway, because they are already stored in iCloud. The device backup on a computer with iTunes or the Finder can include photos, even with iCloud Photos enabled, but if you are using "Optimize" the full versions of your photos are not all on your device, and the backup on the computer will also no longer include your photos. You will have to very careful, never to delete accidentally photos that are important to you, because they are no longer included in the backups. How to back up an iPhone, iPad, iPod Touch be able to Recover Accidentally, Permanently Deleted Photos - Apple Community