I have used a Multitool with those diamond edged semi-circular blades to make capping wide chases for mains cables. The Multitool is much slower than an angle grinder but much less dust, it produces much neater edges to the chase and does not disturb slightly loose plaster too much. Once the edges of the chase are thus marked the rest Inbetween is simply chiselled out. I find this method produces neater chases and disturbs the plaster less.

Having once produced chases through tiles with an angle grinder I would never use one again for this type of job, the dust produced was horrific and in a matter of seconds in the confines of a bathroom I could not see what I was doing.

When putting some conduit up an outside wall of my daughters new build I was obliged to cut away some of the rough cast facing on the stonework to avoid bends in the conduit. I used the same tool and method, again much slower than an angle grinder but far more controllable.

Richard