Coco coir is a nutrient-less growing medium with good moisture retention that is favored by high end pot growers due to the fact that they can control exactly what nutrients the plants receive. Pots and growing medium are discarded at each harvest due to the buildup of salts from chemical fertilizers and to help keep a clean environment as a method of disease control.

The first thing I would suggest is asking what nutrients or fertilizer your grower is using. If they say Foxfarm, General Hydroponics, Advanced Nutrients or any other name brand you can be assured that everything is soaked in petroleum-chemicals. If you are trying to stay natural that would be a deal breaker for you.

If however, the grower is using a natural system such as aquaponics, compost tea, etc. or you don’t care you are still limited.

As you said, coco is slow to break down, its main uses for the average gardener are seed starting, a soil amendment for moisture retention and as a soil amendment to improve texture. As a mulch, I’m afraid you would end up with poor results.

If I found myself in the same situation I would make biochar with it. That stuff is black gold in the garden and if you do enough volume, maybe even a profitable side gig for you. Coco biochar is used in many industries and several scholarly papers have been written about it.