The FCD rear biased bolt catch with their standard length ambi mag release is a good choice. They are designed to work with each other without interference. That is what I used on a recent rebuild.
As far as the upper and lower receivers, Aero Precision is a good choice as well. They are well made, to spec, and finished extremely well. In regard to the LPK and trigger, CMMG makes a really nice LPK and you can get it without the FCG (trigger, hammer, and disconnector). AS far as the trigger goes, I would highly recommend the LaRue Tactical MBT-2S (2-stage) with either the straight or curved bow trigger.
On the barrel and handguard issue, I would stick with the Spikes barrel you mentioned, but use a low profile gas block. As far as the handguard, I would take a serious look at the LaRue Tactical LAT. It is available in lengths up to 13.2" and comes in at $10 per inch. They are very robust and rock solid.
https://www.larue.com/products/larue...ny-handguards/
I have one of these on a build I did almost two years ago. This also has the LaRue Tactical MBT-2S with a straight bow trigger and the LaRue Tactical 16" PredatAR barrel. I also used an Aero Precision upper receiver.
Those are my thoughts on the matter.
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