Anti Virus Software is reactionary. Meaning, as new threats, exploits and phishing processes are detected, the AV software gets updated to react to the threat but usually only after they've been circulated and so there is often a small window of opportunity for the malicious activity to do it's worst. Defender is usually updated very quickly but not immediately which can be said for all AV products. Therefore, the best course of action is to practice safe internet browsing and computer use. Its probably not much help to list the practices i use such as virtual machines, as its far beyond the scope of the question and you can simply google "how to browse the internet safely" etc
In my opinion, you're better off sticking with Defender and just browse safely, taking note of links, extensions, email attachments etc.
Nothing is 100%.