I think maybe we should put hold off on the 'pagan' label for a second here, and switch to a category which is more like 'pre-christian symbolism.' Christianity is a newer religion, right? Yes, so it makes sense that that that religion, or system, would be more filter-like, as we can surely all agree (I hope?) that is it is newer, and so thus would be positioned downstream of an awful lot of symbolic content, and general content.
The problem with this line of argument is that almost everything can be said to have some form of pre-Christian symbolism, or any holy day was close to some other festival in some other culture so everything can be said to be "stolen from the pagans".
People then take the entire history of the world, identify something with "Pagan" symbolism and say "the Christians copied that!".
So you get these ludicrous arguments that Middle Eastern Christians were copying the festival of a Northern European goddess, or that Northern European Christians were copying some Middle Eastern goddess simply because some words sound vaguely similar. Or that Germans in the early modern period start doing something with an obvious connection to their everyday environment, and people claim it actually is the continuation of a "Pagan" tradition from 1000 years earlier, that has no evidence of ever existing, but was definitely stolen as a marketing ploy
The is a giant grab bag of "Pagan" history, much of it not even real, but made up in the past 200 years for people to appropriate simply to reach pre-ordained conclusions.
All kinds on "pagans" came up with all kinds of symbolism relating to their everyday environments. They didn't all simply "plagiarise" each other, they just did what humans do: make meaning from their experiences.
Many of these argument seem to rely on the idea that Christians were incapable of creativity like other people, they could only steal and appropriate with devious intentions. They are usually not an attempt to actually try to look at what may have happened but to paint Christianity as purely derivative because of prejudice.
Undoubtedly some aspects of Christianity were influenced by pre-Christian belief systems. No one ever seems to talk about the areas like middle/neoplatonism though, just some crude myths about Easter being Eostre or Christmas being Saturnalia despite both being obviously false.
I understand you are not making the same crude arguments, but what you say is still relying on the idea A came before B, therefore B is derivative of A.
People can create their own symbolism from their own environment though. A child can link eggs with birth or 'mad march hares' with spring without some deep complex theology behind it. People with lots of eggs can create symbolism about eggs simply because they have lots of eggs rather than because it is a hidden ancient mystical tradition.
Unless there is good reason to believe that B really is derivative of A, then it's just confirmation bias to assume that it is because it matches what we want to see as true.