How about Forsythia...can be pruned to desires size?

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Susanne

Cassia Alata...Candlebush

Cassia Alata...Candlebush

yellow or gold Four O'Clocks, cassia alata (emperor's candlestick), cosmos, dwarf sunflowers, marigolds, yellow roses (my texas is showing here), yellow daisies, gaillardia.

yellow verbascum mullein

Kerria Japonica?

Hibiscus rosa-sinensis Chinese Hibiscus:
Accra 6-7 ft golden yel w/org centers,
Butterball, 4-6 ft, double pure yel,
Fullmoon, 6 ft, double pure yel,
and others, some with contrasting eye.
GH

Here is a link that might be useful: Hibiscus Forum

I know you didn't want a sunflower, but Heliopsis Flore Pleno is a bright yellow, long blooming double sunflower but looks like a mum the flowers are so dense. It goes to about 5' high and as wide as you'll let it get - you can divide it periodically. I was very pleased with my first yr plant - the flowers last a very long time and the plant keeps blooming if you deadhead.

Helianthus Lemon Queen, Giant Cephalaria, or one of the yellow baptisias are what first came to mind, but my idea of full sun plants and Zone 9 full sun are probably very different things.

They're not very exotic, but I love my yellow bush daisies for their flowers and the fact that they're practically care-free and thrive in the sun.

How about a yellow climbing rose?

How about 'Honeycomb' butterfly bush?

Scottish broom (or is that a common name for something already mentioned here?) Suz