Well, first of all, hello! Lump charcoal is hard to buy in my opinion because there are so many different makers and the product is so different from bag to bag. I've tried so many different types. Some brands like royal oak seem to be better than others. It has more medium to big pieces instead of small "unusable" coal. Cowboy brand (for me) seems to have much more small stuff and is not consistent from bag to bag. So, you might take that into consideration.
If you are using a chimney to light your coals, there is no need to let the coals ash over because with lump you don't have all the binders and fillers in the product which is what I understand you're burning off when using briquettes. My point to this is if your letting it burn down for 25-30 minutes, you're probably burning up all the coals on the bottom which would cause them to fall between your grate. Just light your coals and when you see flames coming up through the chimney, dump them. The top layer will still not be lighted, but they will catch quickly.
Hope it helps and good choice with the 18". They are my favorite.
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