I built a lightweight a few years ago, gun came in under 5 lbs with irons. Best bang for the buck part is a slick side upper and a polymer dust cover. That saves weight for almost no cost. The BFT minimalist is the best stock choice for weight, once again not much money over what you might normally use. If you are going to use a lightweight BCG, get an adjustable gas block for the reasons mentioned above. Any titanium parts you add will cost way more than steel, so unless you are looking to get every 0.1 of an ounce out, use it wisely.
Also, look at what V7 has to offer for a lot of your smaller parts. Aluminum is lighter and cheaper than titanium, so if you don't need the strength use aluminum parts (castle nut, end plate, take down pins, mag catch). 7075 is similar in strength to basic steel, so don't think of these parts as super fragile, but they are soft. I looked at a carbon fiber handguard as well, ended up with the v7 hyperlight. 11" rail with full top rail and m-lok or keymod, 5 ounces with the barrel nut. It is a lot more flexible than most carbon fiber options with minimal added weight. Watch your muzzle device choice as well, plenty of heavier than they need to be options as the end of the gun you will feel the most. Avoid titanium muzzle devices, I had one and it sparked like crazy (fireworks at an indoor range). Good luck with your build.
Edit: I went with the 16" barrel to avoid welding my muzzle device, there is less than an ounce difference between the 16" and 14.5" barrel so something else to consider.
with a red stripe on the bottom
