Here's how I would approach that:
RO water, yeah, you are going to have to replace the Ca and Mg even in soil.
Some ferts DO have calcium in them. For example, Neptunes HArvest Fish Emulsion has a Ca value of 1%. Bone meal is also a good source of slow=release calcium if you are doing an organic soil mix.
But I'd stick with calmag for simplicity's sake.
Have you thought about coco? mmmmmm coco