Not sure about perennials that would give the Spring bulbs space but there are plenty of things that might self-seed or have easy to collect seed. Nasturtiums would spill over nicely. Nigella has nice delicate leaves. Poppies sowed successionally if they're brief flowering types. Or there are a lot of nice 'wildflower' mixes

I have a small campanula 'Birch Hybrid' that has self sown into pots with tulips and another with daffs and crocus. they trail over the sides. Sedum do well too but watch out for vine weevils.

I also have hardy summer bulbs in pots along with spring ones, Galtonia, Eucomis too. There are self sown snapdragons that come every year. also consider hosta.
I also have annuals eg, nigella [love in a mist] limnanthes [poached egg flower] that self seed year on year.

For perennials, try the sprawly alpine types. They tend to have shallower root systems. Helianthemum might work, or alpine gypsophila, the dwarfing dianthus, violas, lewisias, house leeks etc.

Thanks every one...nasturtiums already done...nigella I like the sound of...& shallow rooted alpines .