Coco/Red-lava/OC+, CAL-MAG

Planning for a small outdoor or covered patio grow. I got enough Osmocote+ from a friend for my purposes. Instead of buying or making organic soil I am going to use what I have for free. My plan is a base mix of 66% coco coir + 33% red lava rock 3/8'' with Osmocote plus. I have also heard that osmocote is light on cal mag so I can splurge on a bottle of that to have on hand. What do you guys think about this mix, would you make any additions? any feedback would be appreciated. Saving my money for a mildew and botrytis regimen, mid flower is always terrible here.

It feels like a total abomination of a soil mix, especially coming from someone that has made their own worm castings, compost and soil. But it's the only way right now for this project. These will be in containers with saucers to catch run off and no osmocote run off will be leaching into the ground.

Only advice I have is to wash the deuce out of the lava rock.

My thinking would be to add 10-15% humus, whether topsoil or the like. Gypsum is a cheap calcium amendment. Have you thought of a mild measure of lime to raise pH into mid 6?