I feel like there are better ways to make it hard to push malicious updates, while still allowing a user to flash their own devices.

For example: manufacturer bakes in their public key and a per device public/private key pair. The bootloader checks firmware updates against the manufacturers public key and the per device public key. The per device private key is only readable with hardware access via serial or USB etc. The user can extract their device’s private key to be able to sign their own firmware updates. Additionally, the bootloader could support adding new public keys to verify firmware with, so long as the payload to add the new public key was signed by the per device key. This would simplify getting updates from e.g. OpenWRT if they have their own key pair they sign with, vs requiring each user to sign each firmware update with their personal key.