If you go the route of adding an inline adjuster, be sure and put a good quality one. In setting up my touring bike some years back with a rd without one and shifters that didn't either, I made the mistake of using a low quality inline one, and had ghost shifting issues that I kept thinking were related to my genevalle shifters.
once I figured it out, and put in a good one , all was fine.
also be sure to place it so the housing won't force the adjuster if the housing shifts down a bit and then if the bars swing, can force and kink it. I've had this happen once, full handle bar bag not helping with extra force. Learned my lesson with that also but will change it only when I change housings one day.
My SLX 9sp shifter has an adjuster and I have a couple Jagwire metal inline adjusters. I was going to put it about 50-70mm rearward of the chainstay cable stop.