Hmm I'm not familiar with Datomic but in SQL you can always break tables apart as it makes sense. NULL values are fine too if you've already decided that is a valid state in your schema but you can also use mapping tables instead of nullable columns.

>client only declare/specify/depend-on a partial schema that only lists the attributes it needs.

Can you describe what you mean here? It just sounds like specifying your selected columns to someone who is not familiar with the tool.