This is how I just installed my RB Evo on my 2019 1260 Pikes Peak. I run the seat in the high position and have a Shorai lithium battery. I place high strength Velcro on the back of the Shorai and removed the battery strap. I have clearance for the RB, but I still may adjust it bc I don’t like how the wires are facing up toward the seat...Sorry, I have to disappoint you. The solution Yaman gave you does not work on 1260. I believe, the picture that he sent is of 1200. That harness is much longer than 1260 one. There is no way to run the main connector cable as shown in the picture. 1260 is too short for that. You can visually compare the length of the cables on Yaman's picture with the one from the installation manual.
I had to make a new battery strap in order to fit Rapidbike. So, to summarize:
1. Rapidbike (as it's shipped) will NOT allow to put a rider seat in a lower position.
2. You need to fabricate a new battery strap to fix the issue.
3. Reposition bike's ECU on the new battery strap (or whatever it is, attached to the strap) closer to the right side of the bike to free up some space for Rapidbike.
I wish, they make the harness just 20 cm longer and that would be enough to put the device under a passenger seat...
Attached are a couple of pictures of my installation.