Feeding frozen food twice a week isn't bad, but as you said—it'll also eat whatever you feed your fish everyday. I lightly broadcast feed my tank, twice a day, which is how my nem, corals, fish, and inverts all eat. I also dose Red Sea's aminos and broadcast feed a concoction of different coral foods, once a week.
Your light schedule seems fine to me, but again—get a PAR reading if you can. I run my light from 5PM-12AM, no ramping, mostly blue, around 70% output, through both a Top Lid and tons of surface agitation. My nem realistically gets between 200-250 PAR and moderate-high, random flow. When I first got it, it was like 3" and struggling, now it's like 6" and thriving.
I've had good luck with Reef Crystals—a five gallon bucket usually mixes for me at 9.3-9.5 dKH at 1.025 sg. I maintain my dKH level, as well as that of the other trace elements, with a daily dose of Tropic Marin's All-for-Reef. I've had success raising my tank's dKH level with Seachem's Reef Builder—maybe try using that in your tank, as well as in your new saltwater-mix, before performing a water change. If you do go with RB, make sure to match the bucket water's dKH to the tank water's dKH before adding it to your system. Do you dose? You can hold alkalinity at its new level with some sort of daily dosing, like AFR, etc.
How often do you perform water changes, and what percentage of your total water volume are they? What type of filtration are you running on this tank?