Plumbing question- PEX

OK, Long story here made as short as possible....

New owner of a house built in 1949. This is a single story ranch on a slab with a decent attic.

I am remodling the bathroom and found the galvy hot water lines (coming up through the slab) into the bathroom have been replaced with copper lines coming through the attic. I don't know how they were disconnected or which way the original galvy lines are routed. They are not capped in the bath wall so obviously somewhere else there was a problem at one time.

So... as I remove water valves I discover that my cold water galvy lines are showing their age with rust build up and I decide, after much discussion, to run a new cold supply line (PEX) from the laundry room to the bathroom via the attic. In the laundry room there is a 3/4" supply line coming in and the washing maching, outside spicket and hot water tank run off of it. I figured I would "t" into this line to run my bathroom cold water line.

My problem is that today I discovered that my 3/4" supply line in the laundry room is actually 1/2" line which adapts into 3/4" about 2 inches from the slab, then back to 1/2 into the water heater.

So, other than spending thousands on a plumber I have 2 choices; I can stick with my plan and run the cold water line through my attic and into my bathroom (sink, toilet and bath/shower) after I tee from my laundry room OR I can just replumb the bathroom with fresh line and use the existing galvy that comes up from the slab.

Which should I do? Thanks in advance for any info.

I do wonder if my bath hot supply pipe originated from the laundry room??? I don't know these things.

Is splitting one 1/2" line into one cold and one hot 1/2" line going to reduce my volume too much?

edited to add: Home is on city water supply.

How hard would it be to get rid of the galvy pipes in the slab and re pipe accordingly? Its not gonna get any better. Have at least a 3/4" supply from the city would be best.

The volume thing isnt a huge concern, you will notice it if your washing machine is filling up and you are having a shower or flush the toilet, but its not that bad to split 2 or sometimes 3 fixtures on 1/2"(for DIY, anyway)

Pex

Now, what good would it do to leave the underground galvanized lines connected and run a "bypass" overhead? If you run a new PEX line, DISCONNECT the underfloor one so it does not leak someday even though you are not depending on it.