Your gun looks like it already shoots pretty well with your custom loads. I'd say just go and shoot farther out. If your scope isn't up to snuff, that would be the thing to replace.
Usually what I find is when shooting farther out, my auxiliary equipment becomes my limiting factor. For example, when I started shooting at 200 and 300, I found my $99 spotting scope wasn't good enough to spot hits and that was what I needed to upgrade to take my game to the next level. Also get a Chrony so you can measure the velocity of your loads. Once you have that information, you can plug it into various ballistic calculators and figure out your dope without too much trial and error.
I've gone done the route of building multi thousand dollar tacticool rifles with multi thousand dollar scopes but the real limiting factor was again me and my auxiliary equipment. And building new equipment resulted in time lost relearning how to shoot the new gun and developing new loads. Looking back, the incremental return on money spent wasn't really worth it.
I'll bet you if I was using my spotter and Chrony with your rifle and load, I can be ringing the gong at 600 yards in very little time.