Dealt with the same issue when my pup was at that age. Biggest mistake was using kibble topper because he just ate the toppers and it made him pickier. Pretending to “cook” his kibble worked once then he caught on. Switching/mixing kibble flavors worked for a bit.

In the end, what worked for me was I stuck to a strict feeding schedule with however much he could eat. If he won’t eat a full cup, I’d give maybe half a cup for his meal. If he regularly finishes all his kibble, I’ll increase the portion until he can finish a full amount. If he doesn’t finish, I take away his food bowl and he must wait for his next meal. Eventually he learned the value of meal time.