Keyboard not working, but backlight + trackpad work fine

My MBP 14" (2021) has an issue where the keyboard is not responsive, except for the power button.

The backlight on the keyboard works fine and so does the trackpad. The keys are not responsive however.

Running diagnostics (using an external wired keyboard to boot in) shows no issues.

I have read that other people resolved this issue by replacing the trackpad cable, is this likely to work? Also, If I carry out this repair myself, will it be obvious to future Apple techs that a user did it, causing them to refuse future service?

I will take it to the Apple Store first but I am concerned they will charge a huge bill to replace half the laptop!