Save the money on the sensors!
Im a very visual learning person so I made a diagram to explain my watering philosophy so that maybe it's easier to understand.
The plants roots look like a mirror image of what's above ground until the confines of the pot take over. At your current stage of growth it looks something like what I drew below. The most important thing to remember is that the growing tip of the root is another apical meristem just like the top of the plant. It's loaded with stem cells and hormones and is the "boss" of the root zone.
The dotted line represents the 2 inch deep mark and the wavy line on the bottom is where your excess water will settle. If the root meristem is constantly sitting in a wet, low oxygen zone, it will rot and die, effectively topping your roots and stunting the plant because there's not enough roots to drink it away.
Without factoring in the weight, it's possible the top 2 inches are dry but the tip is drowning in a small layer at the bottom of the pot. The top layer has the highest concentration of roots at this stage. That, combined with evaporation from the atmosphere is going to dry it out faster, but your roots need O² as much as they need water.
The arrows are to illustrate why we water in a ring around the edge and never against the stem. You want the roots to expand into new areas chasing that water. The deeper you go, the less roots will be on the sides of the container.
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