Certified GKH Gardening Expert
It sounds like the mixture contained quite a bit of peat moss. This will harden like concrete once it dries out completely. The only way to keep it from doing so is to keep it moist.
From here, I would have to recommend tilling in organic material, such as regular, unused compost. Though mushrooms don't consume much of the nutrients present in those mixes, they don't usually contain a lot in the first place. By the time you go to use them, they are only useful as a way to break up muddy soil textures.
Once you till in organic matter. (I recommend standard compost with a saturated and rinsed (and expanded) block of coconut coir. Coco husk contains natural wetting agents that will help your soil retain more moisture)
If you don't have any important plants in the soil yet, then you can use boiling water to kill off any weeds in the area. Just do this once or twice per day for a couple of days.
These articles will help you to amend soil: https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm