I tried but didn't find any more relevant meanings to the flower, most of the words they use have pretty much the same meaning of the ones I already said before, the rest of the meanings are pretty much exclusive to chinese culture, if anyone want to have a look I recommend search for plum blossom, the other names of the flower don't really give much results, but I think we can safely assume Sumin chose it because of how it's related to winter and spring since it's the meaning and simbology everyone knows, we just don't know the exactly context she uses, I also think that the meaning Martian found is probably another reason Team Gaji chose that flower," "It's never too late to make amend" and there are also other meanings like infidelity, unfaithfulness etc" fits too well with the story to be just a coincidence.
I think that "Three Friends of Winter" could be a possible reason that Team Gaji chose that flower, but neither Seju or Sungji are associated pine or bamboo, same "Four Gentlemen"/"Four Noble Ones" if we put Nami in the mix, so I think we can ignore them for now.