My Dracaena marginata is watered once a week when temps starts to go warmer, usually as Spring arrives. I do the watering always early part of the day so it does not stay soggy wet by nightfall. Typically the signs of over and underwatering will manifest similarly on the leaves, as it dries out and drops those leaves. Sometimes when it is too overwaterd, the stem will get too soft and mushy.. For the most part, it seems happiest being kept on the dry side. It becomes nicely drought tolerant as it continues to acclimate and grow indoors. During winter when most indoor plants slowdown, it can go once every two weeks.
Lava rock is good, it has little holes all over, so roots can breathe below soil level. The plant will also naturally drop the lowest/older leaves first as it continues to get tall, so do not be worried it is doing that especially as the seasons change. It is part of its growing cycle. When it gets too lanky, you can cut off the tall stem at the level you want, but I usually do that in mid Spring if I have to.