Like everything else,work on the observability first.

Can you grab the current boot partition? Once you have it can you decode it? Do you have a reference boot partition? Can you extract the bootloader from the boot partition? Can you read those binary files? Maybe turn them into readable assembly?

Can you clip a multimeter onto a PC trace? Can you do the same with a scope? Can you decipher what the 'scope capture means? Maybe use a bus pirate instead?

It's all about observability.