I have an antique style door lock that a thumb lock on the inside.
Similar to this:
Does anyone know of a smart lock option that fits over the thumb turn? All our external locks are keyed the same so I don’t really want to change the cylinder.
Well, if the lock and the key(hole) looks like the picture above, it should be no problem at all to get a cylinder that works on your current key. I would be cheaper as well as oppose to your plan ‘B’.