Question Placing Foliage and Trees on Custom Mesh

Hello,
I need help figuring out a new method on how to place the foliage and trees on my terrain because right now some of the grass assets don't rotate with the slopes, and some assets are even floating. This has been a problem for those giving feedback on my game.

My current method:
I made a floating island in Blender, I separated the top surface of it and imported both the top surface and the floating island body into Unity. I used "Convert Mesh to Unity Terrain (lmhpoly.com)" to help me convert the top surface into terrain data so that I can use the trees painter to place all of my foliage. I make multiple terrain data copies so that I could layer my foliage as one layer with grass another with trees, another with rocks, etc... And for the most part it looks ok, except for when it doesn't lol. Also, I remove the material on the terrain data so that it is transparent and make the plants look like they are on the ground.

The reason that I did the layers is that I want to make a lush forest and I wanted different densities based on the type of foliage. Is there another way that I can paint foliage onto my custom mesh?

Here is a link to the game but so far people are noticing the problems just through the screenshots: Big World by Littleman27 (itch.io)