Ok, ender chests exist, so why not just keep your diamonds in there, well there are cases where they can't work, like a cash register at a shop.
So if we have 100% unbreakable blocks (not talking about netherite or bedrock, both are technically breakable), seems that would be useful for this purpose but you'd still want them to be movable by pistons so you can get into your vault. Then anyone could come along and move them using pistons.
I've seen in another discussion, that you could use netherite and have elder guardian nearby (it would make most sense to have it within the fortifications). This seems like a great idea except again, the netherite is movable by pistons.
What we really need is NOT an unbreakable block that can be moved. We need vanilla anti griefing mechanics in vanilla minecraft. Just make them incredibly rare and difficult to obtain. Give them very vanilla conceptually and in terms of lore, mechanics etc.
Any one have some good ideas? What comes close to a vanilla anti griefing feature in modded servers these days?