Al Qusais older flats, the plumbing keeps playing up (pressure, scaling, slow drains), what is worth sorting?

You have it right that in older Al Qusais blocks a lot of this comes with the building's age, but most of it is very fixable rather than something to live with. The recurring items are: variable pressure from tired shut-off and pressure-reducing valves and scaled-up pipework; hard-water scaling on aerators, mixers and the water-heater element, which steadily chokes flow; and slow drains from years of grease and build-up in the traps and stacks. A plumber servicing the flat descales or replaces the aerators and worn valves, clears the drains properly, and checks the heater, and that alone usually restores most of the pressure and flow. What is a bigger conversation is genuine pipe corrosion in an old riser, or a pressure problem that is building-side rather than in the flat, those need the building's plumbing looked at, not just the apartment. So the sensible approach is to get a plumber in Dubai to service the flat first, descale, clear and replace the worn bits, and only escalate to building-side pipework if the pressure or corrosion points there. Most of what you describe is a service, not a rebuild.