Water pressure drops bad when using 2 appliances
I would look at your pressure regulator. The device between the meter and check valve. Since it sounds like your pressure is good it could be debris built up limiting the flow. I have worked on many homes with 1/2" plumbing and a toilet should not take a sink to a trickle in fact 2 sinks open should not affect each other as the orifices in the valves are much smaller so I would look to find what is limiting the flow and in my experiance this has been debris built up at restriction points.