Hm. The thing is, I highly value compatibility. Let me give you an example.
Back in the days there was a time when several PC systems were competing. So there was Windows, and OS2 and a few minor others, all trying to get their share, some arguably being better than Windows. But the result was, everyone had files incompatible with other people, so people were unable to share texts or pictures or anything. It was a mess.
So I must admit I am sort of skeptical with the idea setting something in movement like this, because even with the great intent of being "more like then", it may cause
A) fracture of various groups following different holiday times, we suddenly would have one only for the Roman Republic group and then other groups following those of Wikipedia.
B) it may also cause a possibly never ending debate on the "correct" dates, everyone will pull up other sources and we may face decades of theological debates on the "right dates".
So while I understand the desire of authenticity, I would rather argue not starting anything that might even further split our already small community, by starting a sort of "Group Internal Special Calendar", or trying to start a crusade for "the right dates". I understand the desire behind it, but I would still side with the practicality of now established dates as they are now commonly spread. If we open this, when would it end? Who would be the "holy congregation" that has the authority everyone outside the RR would listen to? Honestly, I understand the scholarly desire behind it, but I am not fond of this idea for the sake of practicality. It has the potential to split our tiny community into a myriad of 5-man-groups each with a different calendar.
I would have to see it in reality. But I still side more with the idea to just accept the status quo of the holidays than start a debate to where I see little possible gain from, tbh. I still not see why we should do this.
Almost the entirety of Christian or Islamic holidays are purely made up. Did Christ be born on December 25th? Doubtful. So is it fantasy? It is how history works. Things change. New standards get established.
Opening that debate looks like a potential bomb to our already small and conflicted community. I would want a really good reason to side with that, before I support a move that from where I look at it, has such great potential to split our community. I can see a debate about the real dates only possible when we have a real global community and grew above this level of many 1-2 man groups. If you want to alter the dates, I suggest meeting with actual historians and then start an academic project, at whose end a change of what is written in Wikipedia would be. Making a private projects seems a bit from the wrong angle. I am not per se against changed dates, but I would like to see that coming as a result of academic research rather, in the formally correct way of things.
And one final point. In the end, any religious date usually started with humans making a decisions. There was some day when Roman Citizen X said, ok from this day on, this day is the Festival of Horses or whatever. I just fail to see why random unknown Roman Citizens X's word should be Moses for me.
EDIT: I see holidays and ritual habits more as a bureaucratic formality, tbh. Human beings decide they are so and so, and then we form a community by sticking to the common standard. A bit like the "Social Contract" of Rosseau and Locke, just in religion. We the human beings put the System in place, but it is not a Leviathan that has a life of itself, we just put the systematic of dates and habits there. And like the Social Contract, I see it as negotiable, not as "the one true revelation" that has to be reconstructed. I just fail to see ancient Romans as Saints whose word is law. I see them as role models I inspire to. I am inspired by Cicero. But I am not Cicero, but my own person. That is not "made up fantasy", it means to accept change over time, living history. For me, the question if, say, Concordia is the 4th, 5h or 8th of February is relatively irrelevant. I highly doubts the Gods have schedules which they follow. We humans need schedules. I values the communality of the rules, that they bring us together at the same time for the similar rituals.
Advice is judged by results, not by intentions.