There is some confusion here because while you can disable node key expiration, you can’t disable auth key expiration. But that’s less of a problem than it seems - auth keys are only useful for adding new nodes, so long expiry times are probably not necessary outside of some specific use-cases.

Edit: in fact from your original post it sounds like you’re trying to avoid re-issuing auth keys to embedded devices. You don’t need to do this; auth keys should ideally be single-use and are only required to add the node to the network. Once the device is registered, it does not need them any more - there is a per-device key. You can then choose to disable key expiration for that device.