First of all, I am sure there are other ways of securing a pairs of keys. And I will list some of them at the end of that post.
But, if you really want to destroy an SD card and ensure that its content is lost. I think your approach has some flaws.
First, electrical destruction of an SD card is not a feature of SD cards! This means that the manufacturer didn't try that and performed all the test to ensure that, reliably and in a repeatable way, the destruction will work in all circumstances. Thus some lots might be sensitive to your destruction methods, and some other less sensitive.
You may be able to "zap" the SD card driver. But this doesn't mean the data is lost. You will find plenty of resources online that shows how to solder tiny wire directly to the memory chip and recover the content of a broken SD card. They are meant for that. There are micro tests points readily available to perform that kind of recovery on every SD card (mirco SD card as well. But it requires some work to access them below the coating).
Unless you have access to recovery tools, how can you test that your system really destroyed the memory content and is, indeed, unrecoverable ?
Then there is the reliability aspect. You have to be sure that your system won't trigger unexpectedly. In fact, if you are not attacked by any thief, your system should never trigger during its lifetime. You will have to put a lot of effort and testing the ensure that it never happens.
ESD, power surge, lightning strikes, software bugs, all of this can trigger the system. And if you plan to store a bitcoin wallet and it get destroyed. The impact of that failure is not acceptable.
Home made solutions:
- Put the sd card in the middle of an arc of some kV and sustain it for seconds. The SD card should melt...
- Use a very strong spring and some mechanical part to snap the sd card in half when releasing the spring.
- Put explosives around the sd card....
More realistically there are some solutions readily available for your issue:
Store your keys on a secure USB drive. They encrypt their content and store the key for the encryption in a special memory that is wiped if tempered. The content of the flash is still there but useless without the key.
Use a USB key wallet. They are just meant and designed for that.
If you really want to do it by yourself, use a SDcard but access it through a microcontroller that has a secure memory module. There you can store the key to decrypt the SD card content. The MCU is meant to wipe the secure memory if it detects a tampering attempt.