Although my calcium levels are normal, a DEXA scan (a test used to identify osteoporosis) identified that I had osteopenia

Assuming you mean serum levels, that's exactly what would be expected. Calcium is used in a number of cellular processes, and if there is a lack of dietary input, the bones are used as a reservoir.

You might also look at K2. D3 increases calcium absorption in the SI, but K2 is needed to direct it to the bones - and away from soft tissues. It's important for osteocalcin which, in turn is important for aerobic performance. We humans can make K2 from K1 (same ring, different sidechain) but most of us are pretty bad at the conversion. K1 is from the chlorophyl in green leafy vegetables, BTW - so if you're not getting a lot of that it doesn't really matter how good a converter you might be. Mention of Crohn's makes me think there isn't a lot of roughage in your diet (but I haven’t dug into that condition so I’m likely wrong — non med person here). I'd get into which K2 (different length sidechain), but the TLDR is that it doesn't matter a lot.

I came here to make the point that the importance of vitamins is a function of both which vitamin and who is taking them - but I think maybe I made it in the particulars above?