Low water pressure locating regulator
I agree, it's very puzzling that your entire house would lose water pressure after replacing a shower faucet. I'm putting this in as an answer instead of a comment so I can include a picture of the pressure regulating valve (PRV) so at least you know what you're looking for. It should be on the main line, usually after the meter and before any lines branch off.
It's possible that corrosion could be restricting the flow of water into your house which could result in a drop in pressure when more than one faucet/shower is operating. You might check with your water supplier to have then check pressure at the curb compared to the 80 psi you're seeing at the gauge. Typically pressure is 120 to 140 psi at the curb and your PRV should be bringing it down to 60 to 75 psi.