Good to hear abou the feeding my friend. Indeed, Trachyphyllia are one of the hungriest corals known. Even under natural sunlight, they still need over 20% of their daily "nutrition" from sources other than the products of photosynthesis/translocation of carbon. Point blank: over 20% of their food daily must come from organismal feeding or absorption even if you could have "perfect" lighting.
Regarding the skimmer installation, yours could be vastly improved. The problem is that proteins that overflow from above are allowed to stratify and dilute in the extended path of the sump (rather than concentrate and be promptly skimmed in a primary catch basin/partition). And by static water level - I mean a stable, unchanging depth of water to pull from (like a dam sealed into the sump as the first overflowing partition.
Fluctuations in water level however small affect the head pressure and subsequent flow rate of already variable mag drive pumps that feed our skimmers. The sligt variations in water flow affect (neg.) the water level and skimmate production in any skimmer... making them less consistent.
And if the surface of that water flow is choppy (dicturbing the accumulation/conc. of proteins) then the skimmer is further handicapped.
I believe the placement of your skimmer after a settling chamber and baffles is a flaw my friend. And your weak skimmate production from an excellent skimmer (IMO) does not discount this belief
It is normal though to adjust the gate valve on even good skimmers several times weekly (especially if they are placed in an open sump - arghhh). It is necessary to maintain a steady level of active foam in the neck, is influenced by barometric pressure from weather fronts (bubble size produced), etc/
Food for thought
Anthony