As for the new Mac Pro that is supposed to come out next year: don’t hold your breath. The current Mac Pro is not a good investment at all, and Apple realized they made a mistake with it. We don’t know anything about the hardware yet, so performance is up in the air.
Besides, it depends on the CPU used, and other factors whether that machine will be able to run Harmony any faster - single thread performance is the most important factor.
A shame you are running Essentials: you are missing out on the particles. However, that is another advantage of compositing software: they all come with very powerful particle effect generators.
Suppose you require some rain in your scene, with random wind blowing rain and other particles thrown left and right. You animate the character(s) and render out the scene as an image sequence. Then you use Fusion to open this image sequence, and place the character(s) in the comp. Next you add the background elements. You add lights. You create animated clouds for the backgrounds, and place those in the comp. Then you add two particle generators, one for rain, one for other particles. And you add an effector that simulates the wind. Finally, you add additional effects, and do some colour grading/correction to blend all the elements together.
And you render the entire scene.
Obviously you could do this up to a point in Harmony, but Harmony is not specialized in comping - it is better done in a dedicated app.
Download Fusion and Davinci Resolve, and look up a number of tutorials to get yourself started.
Btw, you could also download Opentoonz, and use its particle effects to render rain, and then bring that image sequence into Harmony.