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The MERRA-2 down-welling short wave radiation data do indeed account for cloud cover, so it could be normalized in some fashion to compute sunshine hours. But, yes, the ephemeris table approach considers only potential sunshine or "daylight" hours.

The NASA Soil Moisture Active Passive Level-4 Carbon (L4_C) product includes daily, gridded surface soil organic carbon (SOC). You can download the data from EarthData Search or from the NSIDC: https://nsidc.org/data/spl4cmdl/versions/7 While the product is posted at 9-km resolution (900 ha), it is ...

You could compute the number of sunshine hours per day using sunrise and sunset times, for which there are several calculators. For example: https://github.com/arthur-e/suntransit If you want to use NASA data, you could estimate the number of sunshine hours (to the nearest hour) using downwelling, s...