Here’s a quick calculation showing how it is problematic:
Sunlight reaching Earth surface = 340 W (Climate and Earth’s Energy Budget)
Photosynthesis efficiency = 3-6% (Photosynthetic efficiency - Wikipedia)
Human energy consumption = 100 W, brain is already 20 W (Power of a Human Brain - The Physics Factbook)
So we have plants energy per square meter = 340 * 0.06 = 20.4
And from that we get the surface area = \frac{100}{20.4} = 4.9019...
So to power a human you need 5 square meters of photosynthesis surface in sunlight. Compare that to the total area of a human that is 1.9 m^2 (https://www.medicinenet.com/body_surface_area/definition.htm) and you should see the problem.
This has been the point the entire time, you need a ton of surface area just dedicated to be in direct sunlight all the time to get enough energy. And you need some structure to support that surface area so you end up with a problem where you need to constantly increase your size to increase the fraction of your body you can use for photosynthesis for sentient plants to be possible.
I don’t know why I went through the effort to calculate this…