on the umbels, as I discovered this year, letting a couple of parsnips and/or celeriac stay in for flowering the next year produces tonnes of flowers the pollinators love... and so many hoverflies! (Fantastic predators to attract to your garden.)
I put in snowdrops and early crocus to attract in and give the pollinators something in the very early months of the year. They establish and "naturalise" nicely in a sheltered spot.
Flowering perennials like the aforementioned achillea are a fantastic maintenance way to go (achillea can be a bit of a space & self-seeding thug, but is controllable, and worth it IMO), I also love echinops and perennial foxgloves for this. They just do their thing with no effort from me beyond planting them.
They're huge and also a bit thuggish, but cardoons are also fantastic - super structural, and they really stand out with their glaucous look ... bumblebees adore them.
I need to get more flowering alliums in.