For those interested, Stapledon did a similar book called "The Nebula Maker". It was published posthumously in 1976, so not freely available.
https://en.wikipedia.org/wiki/Nebula_Maker
My suspicion matches Wikipedia's: it's an earlier version of what became "Ther Star Maker". In it, the Nebula Maker is explicitly refereed to as God, but with the same notion of an artist learning its craft by doing.
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Dennis