What would you put in these gaps between the rocks? Santa Barbara, zone 10, South facing and sun all day.

here's some natives for this:

Chaparral yucca (Hesperoyucca whipplei) & Duleya lanceolata - for your bf who wants succulents

Prostrate coastal sagebrush (Artemisia californica 'Canyon Gray') - fast spreading, billowy, smells amazing, color contrasts well with the stones. the straight species would work well too but I'd want to see it spill down the rocks

California aster (Symphyotrichum chilense) - upright, amazing purple flowers in summer/ fall

California poppy (Eschschatzolia californica)

Lacy Phacelia (Phacelia tancetifolia) - profuse, amazing flowers. billowy even when it dies back in the summer (seriously it's so amazing to see a lot of them all grey together). annual but re-seeds easily. just about any Phacelia would love it there

bunchgrasses like Pacific fescue (Festuca microstachys) or purple needlegrass (Stipa pulchra, not actually sharp) for more meadowy texture

for contrast there's also buckwheats, Eriogonum californica or E. elongatum are more upright, or E. kennedyi/ E. wrightii var subscaposum are more mat-like

I would go to the Santa Barbara botanical garden and find out which native plants would be best. https://shop.sbbotanicgarden.org they could give you good advice.

Does anyone have success stories growing nursery-sourced native Dudleya in rocky crevices like this?

Rosemary and lavender would be well. Also sage.

Would be perfect for dudleya!

Figure out what theme you want to go with. This space could work great for natives like others have mentioned, but it could also look great with Mediterranean plants that would thrive in that setting. Things like trailing rosemary would look good as they drape over the rocks, which would also lower the temperature in the summer. Artemesia, whether something native like sagebrush or something non-native like wormwood would look great. Billowy grass can work in this space like you mentioned, either natives or something non-native like Mexican Feather Grass. There's honestly so much potential for a rocky terrace space like this, I'm a little bit jealous.

Edit: looks like you've already got some trailing rosemary. It'll look great once it grows out more.

Dudleya! And Lewisia. They naturally grow in crevices and rocks. Start with the smallest you can find. In nature they grow there because the seeds happen to land and germinate there so the less root you are starting with the better.

Santa Barbara daises do well in rocky craggy areas. They’ll often be growing between pavers and sidewalk cracks etc. They also spread and make a lot of nice flowers.

Any stonecrop will flourish

Rattlesnakes

Native flowers would be great or like many others suggested herbs.