Bupleurum has the potential to seed out to almost weed-like proportions so I suspect this would be a very good candidate for direct fall sowing or an early direct spring sowing. Be sure to save seeds or allow them to self-sow, you'll never have to buy these seeds again :)

For an earlier limey-greeny-yellow, I always liked Lady's Mantle. The blooms smell so wonderfully fresh and have that nice frothy look. In my climate they always bloom along with peonies.

No doubt too late for your sunflower question but hybrid sunflowers do not produce viable seed.

I've not grown Blue Diamond Limonium but do grow regular Sea Lavender. It's an easy, never fail perennial that gets bigger and bigger each year. I don't care for the smell of the foiage or blooms, I find it kind of stinks. Dried stems lose that smell.