Im a gnostic atheist, but I’m agnostic (in the literal sense of the word - I do not think knowledge is possible at this time) with regards to a historical Jesus.

I can’t fault your reasoning, though I’d like to throw a counter-argument out there as an alternative. The first thing I would point out is that Dec 25th was held to be the nativity of Sol Invictus, which is considered by some to be the reason for assigning the date there. I believe that’s where we also get “Sunday” as Jesus day, and the halo as a symbol of holiness.

In addition the spring rebirth does align with the semiotics of the resurrection - the dead earth coming back to life is the origin of most of the spring-associated festivals in Europe. Dead Jesus coming back to life, and in Christian teachings ending “death” as a phenomenon for his followers, maps well to that.

Also, I don’t think the southern cross was a big thing in Europe. I think those stars were considered to belong to another constellation (when and where they were visible) until all of the rest of this was settled.