The sources you cited don't quite state what you think they do, and some have also made common mistakes.
Notice how, in the
Encyclopedia Britannica, after it shows the
historical precedent for December 25th by Sextus Julius Africanus, it prefaces the rest of the discussion with
"One widespread explanation" not that this was actually the case. It even follows it up with
"One of the difficulties with this view is that it suggests a nonchalant willingness on the part of the Christian church to appropriate a pagan festival when the early church was so intent on distinguishing itself categorically from pagan beliefs and practices."
In other words the Encyclopedia Britannica is mentioning the belief because of its prominence
but doesn't endorse the connection due to problems with it.
Encyclopedia.com doesn't actually cite evidence for how it is "evident" that the early Christian Fathers were struggling for "psychological supremacy" resorting to "expropriation." (In fact that's the exact
opposite of what the Encyclopedia Britanica states in its conclusion.) That raises
all sorts of red flags and requires some serious evidence. None is given. No primary sources (they only cite tertiary at best). No discussion of any of the Early Church Fathers' writings.
Exceptional claims made without evidence can be dismissed without evidence.
Finally,
Wikipedia. Wikipedia *is* admittedly biased. But that's a systemic problem.

With what you've cited, however: It first states that
the dating of December 25 predates pagan influence (which does not help your case) and then it makes some curious claims including Mithras and other practices. Roger Beck, one of the foremost authorities on Mithraic studies, calls assertions of similarities between Christmas and Mithraism the "hoariest of facts" without any hint of proof. The rest of what you cited is actually in support of Christmas being independent from any pagan festivals with the discussion of differing dates between the East and West. The rest of the
live article on Christmas needs a
serious cleanup, as there are lots of claims whose "sources" don't actually link anywhere or don't actually say what's cited (more of the same sort of myths, too, like that December 25th was the solstice – it was not).