I personally would go ahead and farm a few toons you want for arena if you think it'll get you a higher rank or if you think the meta is shifting against you. If you are focusing on a character or two in this way you can still focus on a few req for jtr at the same time. For example, you coukd hard node farm sion and cantina farm kru for bb8 without them interfering. You can also use extra fleet currency to get wedge to 7 stars while farming for jtr since there really aren't many of her req in that store anyway. But my best advice is make sure you are deviating for a team that will gain you something and not just for random quasi useful characters. Also unlesd there is a character you really want in cantina, I would farm jtr req from there right away. Both vets are hard cantina farms and there are two or three fo characters that are cantina farms so that is the bottleneck. For rey and finn you can hard farm them and buy them from the guild store as long as there isn't gear needed by really good characters.
