It looks lovely wildrose.
I wouldn't bother with annuals, just smaller perennials. Your geraniums will flower again if you cut them back, I'd also go for Astrantia (pink, claret, white) and smaller anemones (such as White Swan), smaller penstemon (all colours), veronica spicata (pink, blue, mauve). There are lots of slightly taller campanulas in blue or white such as campanula persificolia ("fairy bells") though they may be a bit tall for what you want. I particularly like the little daisy with silver leaves called Anthemis punctata subsp. cupaniana - bit of a mouthful but it always looks fresh, you just deadhead to keep it going and chop it back to stop it getting too wide.
None of these get fungal problems in my garden, unlike some of the others such as achillia or phlox.
If you go to one of the large online plant sellers like Crocus you can choose plants by flowering month, colour and size. That would give you ideas, though obviously you could buy them locally.
If you want bits of annual to fill gaps I'd always go for some sort of small violas rather than the big pansies.