Just to reiterate what was stated above, Krux was designed to be a hardware signer for multisig wallets, and relies on coordinator software (such as Specter Desktop or BlueWallet) to generate PSBTs that it can sign to send funds out. It isn't really a 'hardware wallet', it's much simpler.
Is it possible to optionally use it as a single sig device, in similar way like it's possible with Seed Signer device in settings?
Multi sig has many advantage but sometimes you want to avoid complications for whatever reason.
The QR code input method offers a compromise by making the 'login' process a lot faster (near-instant reads once you have the camera lens honed in), but it requires that you create one which isn't the safest thing to accomplish. To solve that, Krux itself can print one out for you when you attach a mini thermal printer to it (probably sounds more intimidating than it is! see here:
https://github.com/jreesun/krux#printing-qrs).
Seed Signer have the option for drawing your own QR code manually on paper with sharpie for example, and I think that is more permanent than thermal printing.