Hi Mr. Gunnar!
Thanks for reaching Microsoft Community. We're here to help and let's work on this together.
I understand you're having issues with your Surface device. Happy to help and provide more options here.
I can see here that you have performed most of the troubleshooting already to no avail still. Thank you for verifying that the keyboard works in UEFI, then this can be due to a corrupted operating system due to an update. The best thing to do here is to perform a recovery image reset already. This option reimages the entire device, including the partition on the hard drive. It is the most complete form of reset. Before you begin, you must have at least an empty 16GB/32GB of USB Flash drive and it must be formatted to FAT32 prior to downloading the Recovery Image.
To proceed, please follow the detailed instructions in Creating and using a USB recovery drive for Surface - Microsoft Support.
Please let us know with how it goes. We look forward to hearing from you.
Regards,
RodelioP.