This post goes into incredible detail and is super comprehensive, thanks! I highly recommend also checking out the original paper and presentation at NSDI '20: https://www.usenix.org/conference/nsdi20/presentation/agache

I've used Firecracker for a project last year and I highly recommend it. Building kernels and filesystems seemed daunting at first (esp. compared to how easy getting started with Docker is) but it's not that hard once you understand the basic concepts.