Hey,
Welcome to the forums!
Getting a high ATAR is great to have more doors open for you at the end of year 12 and generally from a technical score perspective 5 subjects is better than 4 so you have the flexibility of 1 not being in your top 4. That being said, you are likely to achieve higher scores if you are better able to take care of your mental health and spend more time productively on each of your subjects. Even aside from that, your mental health is more important than you ATAR generally and I would encourage making decisions that follow that priority. I would suggest you talk to people who are more familiar with your situation if you feel comfortably doing so (especially your teachers) to get their advice about your situation with your subjects as well.
This is your choice to make, but if you believe that dropping a subject has a strong chance of improving your wellbeing then I would go for it.
Importantly, whatever your decision is I hope you are kind to yourself about it.