sounds like you would benefit from tiered caching instead, the free version selects a CF location closest to your origin to use for a 2nd cache layer before contacting the real origin
cache rules don't accept wildcards, that's a page rule I assume? I mean, as long as its showing HIT/Revalidated/Miss (not dynamic) for you in your own tests, it should be cachable
hmm yea without cache analytics most we can do is guess / see what's changed recently. You could try Cache Reserve as well, but it's paid. Ultimately it is just a cache, and cache will eventually expire, your origin really shouldn't be that slow if you need it to be faster, perhaps you could slap nginx cache or something in front of it, although Tiered Caching should be kind of effectively doing that