The problem with this approach is that the flash chips that get used in SD / microSD cards are really bottom of the barrel stuff. It's going to be painfully slo

The problem with this approach is that the flash chips that get used in SD / microSD cards are really bottom of the barrel stuff. It's going to be painfully slow and is practically guaranteed to crap out on you eventually if you use it as a primary volume for long enough. They're built to be as cheap as possible, and last a few hundred cycles of getting filled with images and erased, not thousands/millions of tiny writes. Raspberry Pi users can probably attest to this.