Hi! Welcome to the forum, and welcome to the community
First important observation: Astral Projection is not only useful to get LoS. It could help also in other cases (spells having a range equals to a determined number of spaces, or spells that can be boosted if you're in a specific sector when casting). Not so sure if the nuance was clear; in case, sorry for being picky
As for your query, the interaction between Astral Projection and Boomstone: it's not possible, sorry. Astral projection allows you to "move" the point from which you "cast 1 target spell". Hence, it must be a spell. Boomstone is not a spell, but, as you stated, it's an item, hence, you cannot use it. In the same way you cannot use Astral Projection for Punching.
Please also note that target is also important: you need the spell to have the wording "target XY". For instance, you cannot play a spell like Yoink in this way. But you can clearly use Astral Projection with Lighning Bolt, and so on.
Hope this helps And if you hav further queries, ask! The first WIz-War experience can be a little messy