Atrial enlargement is usually due to hypertension, often due to mitral valve prolapse or due to atrial fibrillation.
https://my.clevelandclinic.org/health/diseases/23967-left-atrial-enlargement One of the problems with pacemakers if they don't 'pace' properly is that they cause atrial enlargement, but can also cause ventricular enlargement.
Even so, your ejection fraction is decent enough for a man with your cardiac history. The calcium scores can be scary, but they're only part of the picture. You actually want some calcification of plaque because that tends to stabilize it and prevent it from getting dislodged and moving further down stream where it can get lodged and cause an embolic event/stroke. But the score is high, and you should probably be thinking seriously of some kind of therapy. Again, though, your assessed degree of occlusion or stenosis is moderate, and while it bears consideration and monitoring, your advanced age suggests that you are probably in the 'average' range of all calcification for your age (average meaning all people falling below a standard normal distribution curve inside of a standard deviation on either side of the mean). You should receive, or have received, all sorts of medical advice from a cardiologist or an internist.