I can't help you out really. A little princess spirea self seeded in front of one of my lilacs but I love your phrase "no poky thorny stuff"!

Hmmm... not really original but perhaps one of the low sedums, like the red 'dragon's blood' or yellow'acre'?? Or maybe star of bethlehem. I don't have trouble keeping it in check around here since I mow next to it. Good luck!

Rudbeckia (Black Eyed Susan) will meet all your requirements but the one about April blooming (most are mid-summer bloomers). They are indifferent to clay/poor soils, re-seed happily if allowed to, bloom prolifically, shrug off drought once established, and love hot, sunny locations. Unlike many flowers, they will tolerate the alkaline soils your lilacs prefer. As a bonus, they come now in many varieties and color combinations, even the tallest of which do not exceed 24". If you are willing to amend the soil just a little with some topsoil and compost, Gallardia are another sun-and-heat-loving beautiful selection, which will bloom for MONTHS if deadheaded. Both these choices are robust perennials, and both should be cut to ground level after fall [scattering seed heads if you choose].