Will adding a half-bath impact the water pressure in the existing bathrooms?
The pressure will be the same as you have now, if you notice a change when somebody flushes or used a sink that same effect will be there. Stay with the 1/2” pipe and you should be fine, most fixtures have flow restrictions built into them so a 1/2” line will be sufficient. Depending on where you tap the lines, the pressure drop could be more noticeable. For example if tapped close to the water heater then run to the new 1/2 bath would have the least effect compared to tapping the line for the existing bath 50’ away because of the total volume and length of pipe, but the effect of using 2 faucets at the same time depends on the flow. A bathtub usually has the highest flow where toilets, showers and sinks have relatively low flow rates.