I have a Black hole weaponry 20” and have been very happy with it, very accurate. Rifle length gas system and I installed an adjustable gas block. From my experience the Grendel is over gassed, so adjustable gas block and heavier buffers are a plus. Mine shoots the 123gr sst factory or hand load under an inch with out trying to hard. I have found a load with the sierra 107 that shoots great as well, but have not seen them available for purchase for quit some time.
triggers, I would have to ask what is your intended use, Bench rest, Long range? Mine has the gieslle, I have others that have Timney, POF and few others and most seem to be good trigger for what they are. Decide if you want single stage vs 2 stage and what # and watch for a sale.