Ditto on the Fein Multimaster and it’s ilk. Try carbide blades. I too waited far too long to buy. Bought one when I bought my present gypsum lath and plaster home. Let’s you be rather surgical with your cutting saving a lot of further carnage and much less dust in the air. Also nice for...
My house is the same stuff. I like it. Much quieter than regular drywall. Every room in my house was papered over the plaster. Wife stripped it all to paint. I hated to paint over such nice workmanship. Glass smooth.
Ditto other comments on a Fein Multimaster or like tool. Waited years to buy...