You can read this post about path guiding if you haven’t check it already Cycles_path_guiding tests
And you can download the Cycles Spectral branch at [https://blender.community/c/graphicall/Cnbbbc/?sorting=hot]
It might give you other options
I think you received good answers already. Although consider that the more light you have in a scene, the less you see light rays caustics.
I don’t have time to test your scene. It’s my birthday 
Add: Or use Octane or Luxcore, they are free softwares also, and do spectral render from birth
Add 2: if you are not in a hurry I’ll have time in the coming weeks, just please repeat the download link for the blend file