iPhone 15 freezes, auto-shuts down every 10-20 mins

I would be more worried about the malware protections you do not have installed.

re space, you will have to either shovel the silage out of social media apps, or if it is all photos then.

Use icloud correctly for more space

Ensure you are syncing to your iCloud Photos (iPhone, Settings, [tap your name], iCloud, Photos, Sync this iPhone)

In the same settings ensure Optimise this iphone is turned on

If this is the first time you turned iCloud photos on leave it 2 days to sync on a good wifi.

 When you are sure that iCloud has all your images and movies you can backup the phone and erase it to factory status, then restore the backup.

The on-phone images will then be sync'd down from iCloud in the minimum size possible (aka thumbnails). You will also lose excess System data which will have accumulated after 2 years without updating.

Then update to iOS26.3

Then have the battery replaced even if nowhere near 80% with Apple Support's help

Resources needed:

Set up and use iCloud Photos – Apple Support (UK)

How to back up your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Just so you understand, the lack of storage is what’s causing the freezing and respringing of the springboard. When memory runs out and insufficient storage space is available the springboard crashes and you’ll see a black screen and spinning g wheel for 10 to 30 seconds before the process completes.

What is the battery health and why in the world are you so far behind on iOS updates?

If you don't have sufficient space to install iOS 26 over the air, do it using your computer.