How to increase flow to upper water outlets
That's normal in every house everywhere.

If someone's in the shower, don't open the faucet!  That's a basic rule of cohabiting a house with anyone. 

If you really want to engineer your way around that, you'll need flow restrictors on every faucet, and a temperature compensating blending valve on the shower.   How much flow does that faucet need?  You may be thinking "the absolute maximum possible", well, then, OK, you can't use it at the same time as other faucets, because it's taking it all, as you intend. 

Since you've made all the pipes huge, the limiting factor is now the water supply coming in from the city.  

The difference in bar pressure is explained by the difference in height between floors. 0.5 bar is the weight of a column of water 16'9" tall or about 5 metres.