In simple terms, you would need to get the QR code from the third party app to elsewhere, e.g. as a screenshot or printed copy.
But…
You will find that you will need to keep WhatsApp installed on a device. WhatsApp doesn’t ask me just the once, it asks me regularly to log back in to the linked device. You won’t just be able to uninstall standard WhatsApp.
And surely all the benefits of the sandboxed application are lost if WhatsApp “proper” remains installed on the same device?
In my case, I keep an old android lollipop at home without a sim card, and this works for me. WhatsApp proper can track an old phone that never goes anywhere or does anything else.
I will go the route of the old spare phone and leave it in one place . And with only the contacts on that phone that I keep Whatsapp around for. Or even dedicate a new sim/number to it to separate it completely from my Murena FP4