Any way to reference first frame to more easily seamlessly loop an animation?

Hi, I am trying to create a loop animation of a waterfall and so I want it to perfectly loop and look seamless. I am struggling to find anyway of getting the onion skin to loop so that on my last frame I can see the first frame being shown in the onion skin to help me seam it together. If anyone knows whether this is possible or even cloning frames so that I can clone the loop in that when I edit on the first frame it also edits the frame right after the end of the initial loop.

I really hope this makes sense, I'm just trying to make a seamless FX animation loop and I'm struggling to find anything that makes it easy to look back on where particles are on the first frame when I'm working on the last. Any assistance on tools that may help me out would really be appreciated!

I am slowly building up main splashes on the waterfall and their arches and so I am adding more and more which part way through their arches go from the final frame back to the first frame and so I am finding it hard to see where abouts that will be without having to click back and forth between them. Maybe there is a hotkey to jump back and forth between the first and last frames? like < and > skip between each frame.