I am planning G-d willing a surprise birthday Zoom “meeting” for my mother’s birthday. First, I am interested in knowing if surprising people even with positive things is brought in halakha. Also, I would like to ask a broader question regarding secular family relatives who like to discuss very deep, philosophical topics like the existence of the G-d. May I engage in such discussions, or perhaps not and I should suggest other topics in Torah? How should I respond?

ב"ה Shalom I can't think of the issue of surprise in Halacha in particular, but we definitely find a lot of references in Torah sources in regard to bringing to joy to others, besides the respecting of one's parent. In a few weeks we will be reading Parshat Vayigash where Yaacov is told that Yoseph is still alive and the Torah says "…and the spirit of their father Yaacov was revived" (Bereshit 45:27). In Mishle 25:25 Shlomo Hamelech says "like cold water to a tired soul is good news from a distant land." upon which Rashi and Ibn Ezra speak of the virtues of bringing good news onto others. Making other people happy is also brought as a virtue in Pirkei Avot 6:1. The Talmud (Ketubot 111b) highly praises he who brings a smile to one's face. However, while surprising in the positive sense is virtuous, startling is a negative thing both from an ethical point of view, not to mention from a medical point of view. Again, referring to Yoseph, when he divulges that he is Yoseph, the Torah says that his brothers couldn't answer him because they were startled. (Bereshit 45:3) upon which the Rabbis speak how greatly they were frightened and from here they infer how we may approach reprimand from Hashem. In regard to your other question of discussing philosophical issues about G-d; if the questions are sincere you may do so, since it is great Mitzva to instill belief of Hashem in people. For such a discussion, however, you must be knowledgeable and prepared. If you don't how to handle such a discussion on your own, you can say how you personally feel , but you could refer the questions that you think you can't handle to another friend, Rabbi or a book. But if the discussion argumentative, I would not recommend such a discussion, since it usually does not yield any benefit. All the best