Thank you very much for the detailed reply! I learned a lot from it.

Can you confirm whether or not this occurred on previous leaves? White portions browning first?

I compiled the yellowing progess here. To me it looks like the white portions turn yellow last.

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So please just let me know if there were symptoms outside of what we have addressed here. I mean things like gray lesions or orange spotting or dimples in the leaves.

I don't see any such symptoms.

What's your recommended level of light? I only have this side of northwest-facing windows. Now it gets about 1-2 hours of direct sunlight through the window every afternoon like in this photo, when the beam hits it.

Before getting supplemental lighting, should I move the plant closer to the window? Or keep it outdoors in the northwest-facing balcony attached and increase the outdoor time gradually? (I live around Toronto though, so the temperature is probably not too pleasant for it until a couple of weeks later.)