Capers are considered a “fantastic substitute for olives”, but they are already in the recipe! Personally, I think you could probably just leave the olives out. If you search ‘substitutes for olives’, I don’t think I see anything else I would use (pickled onion, mushrooms, pickled carrot, etc.). I did see jarred artichoke hearts a few times & they would add some salty, pickled goodness, if you like them.
For my recipe, I used a can of ‘just olives’ - green with no pimiento. They were really ‘flat’ - next time I’ll use some in a jar that have some pickling aspect.
Some folks hate the prunes ... for those, I saw people substituting apricots and also dates (I love both, but like prunes, too).
Thank you, I think the artichoke hearts might be an option. He’s fine with capers, too. I’ll give it a whirl sometime and let you know how it goes!