They're just scavenging the older leaves, it'll probably keep working it's way up the plant until you transplant whether you feed harder or not. I'd definitely be feeding them every watering now until they go in the ground though.. Have you checked to see if they are getting a bit rootbound?
Yeah I'm very big on supercropping I love that shit lol but I don't go all out training them along the ground I just bend the highest tops into the gaps until they make a flat-ish canopy.
I'd go for the scoria but the coarser 15-20mm grade and I'd buy a bit more than enough for a 2" layer in the bottom of the holes then chuck whatever's left into the mix. Unless I had to go for a big walk through the bush then I'd taper it down depending on how far it is, how big I plan on making the holes, and how lazy I feel. You could just use it like perlite too and mix it in I guess. Maybe do a side by side? Use the same amount for each hole, but mix all of it through some plants and chuck it all in the bottom for the others?
You could also mix in some other stuff like dolomite lime, blood and bone, potash, gypsum, kelp powder, worm castings, and sheep pellets, they'll love all that shit.
I've noticed I've sort of been making myself out to be some kind of master grower since I've been here though so I have to add a disclaimer that I've only grown in the ground a couple of times so far, each time only just growing enough for personal use, and I've only tried that layer method with a raised planter box for some veges but they cranked. Good luck bud keep us posted on the progress.