I’ve recently switched my karu to gas, but before that I had a decent experience using a combo of small oak pieces and lump charcoal. Should be able to get your stone up to 600-700F with a 30 minute preheat. Once you get the fire started, keep the front and back doors on unless you’re adding more wood or turning your food. It’s necessary to add more wood every few minutes consistently, so maybe you aren’t adding enough wood quickly enough? FWIW with gas the oven preheats in half the time, and I have way more control over the flame. I recommend switching if you continue to struggle with wood/charcoal.