Troy you seem very comfortable articulating what you are looking for. My input on this subject is to find the nearest police outfitter in your area and buy a used service weapon. If the gun is available used it will almost certainly be a S&W. Also watch for local gun shows. The 38 spc is by far...
The quick answer is stay with Aluminum. Yes it weighs more than Polymer but it would be such a small difference who cares? However, the Polymer is plenty strong enough so go with easy.
Yes, they sell direct mount key mod grips but a short 2* or 3* picatinny rail section is less than $5.00. But...
The correct solution
I am sorry so many responded to your question and apparently missed the issue.
You need to use one or two or more (?)
shims usually supplied with the hand guard. Your problem is the barrel nut is bottoming out against the upper receiver without pinching the barrel...