I saw some Sapphire type carts on e-bay that are 0.3 ml total capacity. That's a great idea. The 5 ml tanks sure feel a bit like a ballerina in a 13 wide pair of galoshes. But if it's a bottom feeder with no slack at the bottom, it can feed even if it's only 5% full. You could add a couple drops of ejmix and warm it up, or try to stir with a wide gauge needle (18 gauge or fatter) and slowly aspirate into the syringe, pump in and out to mix. It doesn't have to be evenly mixed. You can even put EJMix on the wick and let the oil blend ON the wick. Lotsa possibilities.
Sure, the cold oil sticks to the sides, but if you slowly gently warm it you can get most of it to seek the bottom.
I'd forgotten about this one 2.2 Ohm Master Kush MiniProTank2 lying in the Concentrates Chest, a lot of oil on the glass wall, not a trace on tip of the 510 connector, and so I stood it tip-up for a couple hours, and it was locked and loaded! 3.3 volts on an Ego Twist, two hits and - Wow! But even if I hadn't let it stand, there's always enough in the oven for a couple good hits, the thick oil is very happy on the wick if it's nice and porous, i.e. if you don't burn it.
You'll just have to keep trying things until you find your happy medium. If you have to thin it and even if you end up with 80% EJmix, it may taste like Polly Ethyleen's tit, but it's still gonna getcha there, it's not fatal, next one will be better.
I like 70% thick dark oil w/ 30% EJmix in the Sapphire type, it makes it easy to handle, and 100% oil in the MiniProtank2. The MiniProtank3 has that big wick hole that I don;t think is worth messing with. It'll feed like gangbusters initially but then IMHO too many things start happening at once.
With these oils that spec at 70% THC, it's powerful medicine, it only takes 2-3 light hits to adjust me brain. Bystanders don't even know where it's coming from, if you stealth the device and keep the temp low. I toked 4-5 riding the Bay Area ferry, and it was ... too much. The point being that PGld and the thickest oils still feed well enough to vape and not burn if you go slow. If you go too fast something's gonna burn ;-)
Best wishes!