... I am fascinated with the RPR and its "out of the box" readiness. If I bite on that, it will probably be in .243, as I have a strong suspicion that it will be a better long range rifle than the current darling 6.5 Creedmoor. You made the right choice by going 6mm IMHO.
Ruger RPR (stock photo)
I selected the .243 as a long range target and varmint rifle. Varmints here are groundhogs and the occasional Coyote and also to match calibers with my Shaw Precision Custom, which is better field carry than the RPR, which is best shot off my Lead Sled.
I also have a Ruger SR762 "battle rifle" in .308 so the larger caliber rifle is covered. The magazines for the RPR interchange with the SR762 and that's a big plus for me too.