How to properly address a Korean friend in their 40s?

I am a woman in my early twenties, and I have been close friends with a Korean friend for a few years now. That friend is in her forties, married, and has children, and I am wondering how I should address her. When we first met, I was a beginner in Korean, so we mainly communicated in English and I called her by her name. However, my Korean skills have improved, and we often converse in Korean now, and we've become closer. The problem is that I don't know how to address her. I've heard that using '씨' can be considered a bit rude or distant, so I mainly use '님' in messages. She hasn't pointed this out yet, but I worry that she might just be letting it slide out of politeness. Especially, I want to write a heartfelt card to her in Korean on a special occasion. However, I feel that '님' might be too formal and not appropriate. I am curious about the proper way to address a friend who is older in Korean culture. I hope this writing doesn't reach her.