My first thoughts are that you've modified too much in one go.
If I was removing the cacao powder, it would need to be replaced with something in a flour form. I'd likely increase the raw buckwheat flour by the same weight/volume.
I've found that I can easily reduce the sugar content in things I bake without affecting the end product. Usually I reduce by ⅓rd to ½ without issue. But the unsweetened applesauce here will be adding some sweetness to the mix and omitting it will affect the end product because the applesauce is a thick liquid (It still contains water in my experience). Replacing that with a powder or crystal will need additional liquid adding to the mix. How much? No idea. I'd have to cook the original recipe first to see what the cake mixture looks like in the original before I could make changes like that.
Psyllium husk powder isn't a substitute for applesauce. It's a fibre, applesauce is a sweetened sugar carbohydrate made from apples. The 2 really are not interchangeable, at least not to me.
Dropping the oil by 50% will also radically alter the mixture.
And arrowroot is a starch (thickening agent) and often used to replace wheat flour. Psyllium husk powder is a fibre or bulking agent in baking as I understand, again the two don't really interchange. Also it needs water, to expand and bulk out. Adding it will mean that you need extra water, but you've reduced the water content by removing the applesauce.
Typically gluten free recipes don't rise as well, so making a lot of pretty major changes to this recipe isn't going to work well unless you take it 1 small change at a time.
I think you need to start by making the cake as is if you haven't already.
Then change 1 thing, such as removing the cacao and replacing it with raw buckwheat flour and checking that works.
Then you could cut the oil down, but not by half. That's too much of a jump. See if that works.
And so on.
