I’ve always found that the horizontal-stack does centre the cards that are in it, but then I don’t use custom:button-card, so that might be different.
Two years on from the original post, I’d probably suggest the (now, not soo new) grid-card as the container for the others.
Although, this is just a vertical-stack with a markdown for the title and a horizontal-stack for each row.
and it centres the cards fine, no card-mod or anything (not for centring anyway)
What have you got that’s not working?