21st Field Artillery Battalion, 5th ID?

We are trying to piece together my Grandfathers service. We have some pieces of passed down family lore but he would never talk about the war himself. I would like to figure out what is factual and what was false lore if any and basically back up our genealogy project with some sources.

From what I have been told my Grandfather served with the 21st Field Artillery Battalion, 5th ID. I have no idea when he traveled into theater but according to his enlistment date he could have entered at the earliest time periods, possibly even in Iceland although I have never heard it even mentioned. I am getting his military records to know for sure on placing him in the unit as a fact and hopefully theater dates.

The other story we had passed down was he was either the sole survivor in his "unit" or one of very few after being ambushed. I have been told this happened after 25 December 1944 after they moved north out of the Echternach area. I am not sure where this information comes from. It is said that as the "unit" was overrun he had to bury himself under bodies to play dead to survive. I also don't know what is meant by "unit" because I am assuming it does not mean the 21st FAB because I have seen so many other family stories of those that served and survived the war during the same periods of time. At this point I am guessing that it was maybe a platoon or gun crew level that may have been hit hard or this is a misunderstanding through the years of another term I heard thrown around talking about his unit losing 133% in casualties which I am assuming is referring to the turnover of the 5th ID during the war. This is the story I most want to learn more about.

The other curiosity is if he was with the 21st, what day did they land in France? I continue to get conflicting accounts. The only movements online I can see are very broad in detail and mention the 5th ID landed on 9 July on Utah, yet every post I see about members of the 21st is that they landed on D-Day. I just find it odd that every time I see the 21st mentioned about when they made landfall it was D-Day but at the same time, none of these were official sources. I did watch an interview of a member of the 21st FAB and he says they landed on Utah with light casualties but those on Omaha were taking heavy losses and specifically says June. So that is also a curiosity.

So, I am reaching out for any help on good sources that will discuss in good detail what happened to the 21st Field Artillery Battalion. There is entire libraries full of stories about the Battle of the Bulge, 5th ID, and the Third Army but I have no clue which would narrow down to each units progress and shed light of such an ambush and not to mention learn more about his service in theater on a closer in scale. Can anyone make any recommendations on books or other sources or maybe historians for the 21st FAB or 5th ID?

Thanks in advance and I appreciate any help you can send my way.