I don't know about Speedtree but this might help you:
Create a tree in Unity 2018.3 and export it using the Unity FBX Exporter
Try the Modular Tree addon for Blender and then export as FBX
Trees are also created via nodes
Leaf placement was problematic and I encountered a few minor bugs back when I used it
Might not yet be stable in the (fairly) new Blender version 2.80 but 2.79 should be fine
It's free & open source
Also consider using Blenders Decimate modifier to reduce triangle count of your exported mesh to create LOD's instead of doing it manually (Requires some knowledge of Blender or similar 3D modeling software):
- Add a decimate modifier
- Play arround with the 3 modes and their values
- Fix uv's if the modifier messed them up (But: Automatic uv generation in Blender isn't half as good for organic meshes as it is for architecture so you might have to get your hands dirty anyway and fix uvs by hand)
- Apply the decimate modifier
- Edit Mode -> Select faces with wrong uvs -> Smart UV Project.
- Select all uv's -> Average UV Island Scale
- Move new uv islands
Seperate the leafes into it's own mesh object if the decimate modifier only seems to work for either only the leafes or only the tree base (non leafs)