I don't believe people were underestimating them. It shows what an united, organized and ideologically Arab tribe is capable of. And I'm not referring to their attacks on Saudi Arabia but ability to suistain a war economy for an extended period. Iran plays a big part in that, for sure. I think the strange thing is Saudi government's lack of initiative which doesn't appear to me as by design.
And I also don't see what you're seeing that Iran is doing this to 'weaken defenses around Israel' before going in all on Israel. Not seeing any of signs of that unless they pull a trick up their sleeve. Israel is really far from them to do that. And way more capable than Saudi Arabia. Saudi Arabia and GCC are in close proximity.