Blackberry Cake Filling - Testing

I'm adapting the Blackberry Coulis recipe to try and make something more solid that I can use to layer in the middle of a cake. I've used info from the link below and totally changed it. I used Arrowroot powder and only 1oz, and it has made the gummy arrowroot thing. Waiting to see how it works as the filling in a cake with buttercream frosting as one of the layers.

https://sugargeekshow.com/recipe/berry-cake-filling/#recipe

1 - The arrowroot seemed to absorb all the flavor and was too rubbery. Trying again with cornstarch.

2 - The flavor is better, but I'm not sure how to use this. I tried making a berry cake for the Hub (which I wasn't really excited about) and it was just ok. I don't think I applied it well to the cake, anyway.

20oz/568g fresh blackberries

1/2 cup (100g) white sugar

juice from 1 lemon

1½ oz cornstarch

2oz water

1. Over medium-low heat, add the blackberries, lemon, and sugar to the saucepan. Stir to mix well.
2. Bring to a simmer and cook, uncovered, for 3-5 minutes (up to 10 minutes if using frozen fruit to help excess moisture to evaporate).
3. As the blackberries cook, in a small bowl whisk until unform the arrowroot powder and the water. You will have to whisk again just before using because the arrowroot will want to settle.
4. Either press through a sieve or use a blender to crush the fruit and strain out the juices. 
5. Return the blackberry juice to the pot and heat until starting to simmer.
6. Stirring constantly, add the hydrated and whisked arrowroot powder to the blackberries and cook until thickened (this is pretty quick).
7. Allow to cool to room temperature before using.