You might want to set some rules for when the art is past due or how often you'd like an update, or at least a part where you would agree on a communication strategy.

Also you probably need a "failure to comply with these terms will result in ________" where the blank is action(s) being taken

I thought about doing an "if not" section, but I do still want to have artists allow my commission. I'm trying to avoid threatening them.

but a part about updates would be a good idea.

If not section might be as simple as "these terms will be revisited and a new contract drawn up" I guess. :p I don't know much about law or contracts