Is it permissible to say Bismilah in the bathroom before making Wudu? Since there’s toilets in modern day bathrooms? Is it mandatory to say Bismilah before making Wudu?

It is not mandatory to say, but you can say it before entering the bathroom. Even if you say it inside the bathroom itself, it is not forbidden; it is makruh. However, as the ulama have mentioned, you should say it before you enter.