Best thing about ARs...modularity

You could, for about $700 or so, buy a quality upper in .223 or .204 Ruger and keep the lower. If you went that route, you would want to improve the trigger in the lower to get the accuracy of a free float and match barrel, so add another $150 to $200.

Not a fan of most fluted barrels as most are not done correctly and offer no real heat dissipation advantage, some due to the difference in heat profiles can errode accuracy. I also prefer a shorter barrel than 24". 20" is plenty. The longer the barrrel the heavier the profile needs to be to damp out the vibration and that added velocity really is not that big of a deal.

If I was in your shoes, and since there is marketing hype with "M4s", I'd sell the M4 upper and get a quality upper with a 18 or 20 inch match barrel. You'll want a free float, a comp and still get a good trigger in the lower. I've seen some decent uppers sell for good prices in the last few months.