“Are differing tire tread patterns dangerous?”
Probably not. No one here knows for sure.
It might be possible to go out to the test track and create extreme conditions (heavy rain, high-speed turns) in which different tread patterns could affect the car’s ability to manuever. Or maybe not. Since nobody here can give a knowledgeable thumbs-up to your intention they cannot endorse it, particularly since this is a safety issue. Hence the conservative approach to their responses.
My guess? I’ve spent a lifetime driving with mismatched tires. I’ve never experienced a problem that could be associated with differing brands or treads. You will probably do just fine using the Pirelli.