We have 1/2” copper lines in all plumbing throughout our house currently. We will be bringing in a larger 1” water line from the street to current connection to increase flow. I am roughing in a new small bathroom upstairs - would it be wise to use 3/4” pex supply lines up to the new bathroom and then split into 1/2” lines for the vanity, toilet and bath/shower combo?

without knowing how difficult it will be to run lines up to the new bathroom, it is hard to say.

i am a fan of home runs all fed off a manifold with individual stops.
if you can run separate feeds to the bathroom you will have individual shut offs and no joints inside the walls , floor or ceiling.
you are looking at something like this. they are not expensive, save money in the long run and can be had with 1 inch inlet, as many outlets as you choose and continue out the open end to feed your water heater.
you can also place one above the water heater to run some hot feeds.