Storage problems Huawei?

I have a Huawei P8 Lite 2017 and for some time now I have been seeing that my memory is full, even though I only installed Whatts app and deleted almost all pictures and videos or dragged them from the phone. Even resetting to the factory settings could not solve the problem. If I look at the memory, I also only come across about 11 GB of used memory (see screenshot). I have an SD card, but the model is not compatible with an SD card. Is the problem possibly due to the hardware (cell phone about 2-3 years old) or can I somehow solve the problem in another way? Android version is 7.0

Thank you in advance for help and for further information.

Oha. It's already very full. Yes, all images and files are not displayed.

I have had very good experiences with f.stop, or connect the mobile phone to the PC and delete all unnecessary images there. Usually there's a file called "nomedia" or something similar in the folder you can't see.

I've already deleted almost all of the pictures, that's the problem. Although there are only WhatsApp and approx. 20 pictures on the internal memory, it is displayed as full.

Did you also empty the trash?

12.5 GB are occupied by apps and firmware, and when you have emptied the trash, restart the phone.

Yes, I've emptied it several times and also restarted and resetted it.