Crocus supposed to be good under creeping thyme. I haven't tried this combination.

Tulips
Iberis
Helianthemums

If you like stark color contrast another thing that might work is having them interplanted with back mondo grass.

Try snapdragons.

We had a successful show last year with planting started plants of Rocket mix snapdragons in 2 separate mixed shrub & perennial beds out front. They slowed down during our brief summer heat, but the blooms soon returned. Plus the snapdragons aren't devoured up by slugs like marigolds or petunias are here even with pet safe slug bait & patrols. We bought more again this year for $.99/6 plants at McLendon's. Sale goes until Sunday & they had several colors & heights. Harden them off & get them in the ground as soon as you can so they don't putter out in the small pots.

You can't beat the price & late spring until fall blooms if you can provide what the snapdragons need. It sounds like you have the lighting that will produce good blooming & give them a break during mid day.

What was most interesting was that the colors bloomed together meaning yellow from one bed was showy with the yellow in the other bed. Then later it was the reds, then again back to yellow, then the pinks. It was a great show all season long until frost.

When planting out I put one 6 pack in each bed, but did put the darker foliage one of each pack together in the same bed. You can't tell which will bloom yellow or pink, but it worked out. Our soil is rich with compost & we added mulch. The beds are partly shaded by the house from 11:30-1pm or so and larger shrubs also provide some shade. I watered them as needed by hand if we didn't have rain each week, but I don't recall more than a few times all summer. I had to water the veg garden a lot more than that.

Hope that helps,
Corrine