What to plant this year in my flower border?

in Plants
I am trying to grow more flowers and started a border a few years ago. I buy a couple of new flowers each year at Hampton but fear that will not be possible this year so was wondering what next?
The border is south facing at the foot of my neighbours beech hedge with very poor soil which I am gradually improving. I have been buying light coloured flowers to brighten this edge with a few darker ones for variety.
I have:
1. A lovely deep purple hellebore for winter which looks great behind the primroses for spring.
2. The nigella looks lovely through summer and keeps coming back every year.
3. Last year I bought 2 white gaura (although one mysteriously disappeared leaving just a hole) which I am planning on propagating.
4. A purple sage which I use for cuttings for the herb garden.
5. Lovely deep red sedum (or whatever it is now called) which is great in the autumn and into winter.
I am looking for another light coloured perennial that will flower in/from May to fill a couple of gaps between the sedum and the sage. Nothing too invasive but maybe something I could take cuttings from. Any ideas? The more bomb proof the better as I am still getting the hang of flowers and they sometimes suffer for it!

The border is south facing at the foot of my neighbours beech hedge with very poor soil which I am gradually improving. I have been buying light coloured flowers to brighten this edge with a few darker ones for variety.
I have:
1. A lovely deep purple hellebore for winter which looks great behind the primroses for spring.
2. The nigella looks lovely through summer and keeps coming back every year.
3. Last year I bought 2 white gaura (although one mysteriously disappeared leaving just a hole) which I am planning on propagating.
4. A purple sage which I use for cuttings for the herb garden.
5. Lovely deep red sedum (or whatever it is now called) which is great in the autumn and into winter.
I am looking for another light coloured perennial that will flower in/from May to fill a couple of gaps between the sedum and the sage. Nothing too invasive but maybe something I could take cuttings from. Any ideas? The more bomb proof the better as I am still getting the hang of flowers and they sometimes suffer for it!

"The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it." Sir Terry Pratchett
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I am also tempted by the Baptisia Australis 'Alba' a lovely looking plant and I enjoy the sedum over winter so the seed heads are a bonus. Would it manage in a hot border on clay do you think?
I might just risk it, maybe get one of each and see how they go. The gaps are about 20cm diameter so should be OK I think.
The sedum is there to hide the hole in the fence that the local fox uses so I could thin it a bit and move the sage along. Would 60cm be sensible? RHS says 2-5 years for height but doesn't mention spread, I'm assuming they increase together?