How do I recover photos and videos after sudden storage decrease on iPhone 12?

The $2.99 a month for iCloud storage will not help you if you have not turned on storage optimisation in iCloud photo settings. 30GB for photos sounds like you may not have optimised turned on.

Checkout your highest apps stirage in Settings, General, iohone storage. Also checl system stirage at the end.

Come back if you dont know how to eliminate thise.

"How do I recover photos and videos after sudden storage decrease on iPhone 12?: I have an iPhone 12 with 64 GB of storage, for the past multiple months my storage has been in the 55 GB to 63.8 GB range/64GB (so pretty full). I pay $2.99 a month for storage but that has been full for months too due to photos and videos. The past week my phone has taken up around 63.4/64 GB[...]"

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For the reference...

Clear Space:

If it was entirely fully, then it would have stalled, meaning you'd have to reset this to Factory settings. So, to avoid this, see just how much space is taken up by System Data. You can rid of it in-full from an iPhone: How to Clear "System Data" in your iPhone's Storage: - User Tip

Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support

So, what to do...

Clear Space Using my User Tips:

• How to Clear Space from your Mobile Device, and What to Clear: - User Tip
• How to Clear "System Data" in your iPhone's Storage: - User Tip