Q.Did I water my dieffenbachia too much or too little?
After blooming some leaves started to turn yellow and mushy, even though I only watered it once a week. I thought I wasn’t watering it enough and tried to do something about it, but more leaves kept turning yellow.
Then I left for two weeks, and when I came back no new leaves had turned yellow despite having been dry all that time(remnants are visible in the first photo). So just what happened?

Certified GKH Gardening Expert
I would say too much watering, considering the two-week interval did no harm. From the photos, the plant looks pretty good.
Overwatering is the most common problem with diffenbachia. Since soils and pots and drainage are different with each plant, I use multiple assessment observations to get the watering right, physical and intuitive. I test the soil surface with a finger, use a soil moisture meter for deeper soil moisture assessment, and look at the foliage for possible wilting.
Here are more tips:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm