The short version is: your screen is buggered, you need a new device or screen.

Given you dislike of uncertainty and "new things", your options are:

1. Contact Ledger support and see if they'll replace the unit
2. If they won't replace it, see if you can get a discount on a new device from them
3. If they won't do a discount on a new device, then buy a new one full price
4. Buy a BIP39 compatible device from a different manufacturer

I do not recommend that you fiddle about with replacement screen options unless you have prior experience with electronics repair. It isn't exactly a super complicated process (as seen on this video: https://www.youtube.com/watch?v=hnJKEoBHZdY) and doesn't require soldering anything, but it isn't "changing a light bulb" easy.

Having said that, if the device is basically non functional, and Ledger won't replace (or provide a substantial discount) then spending a few dollars on a replacement screen from aliexpress might be worth a try as you're really got nothing left to lose at that point. You just need to ensure you buy the 12pin version of the screen (there are 8pin, 12pin and 14pin variants)