I'm wondering what part of the world your flowerbed is?
I've been researching various things about this... and I can say after researching lavender quite a bit, because i think I might have an area that it will grow in well... An area that it seems not a whole lot I tried last year did take to at all. And only certain things grow in that vicinity, and haphazardly.
This is after considering all of the flowers & plants that seem to grow naturally & well in that area, I figured out why lavender might actually have a shot there - despite many gardeners feeling that lavender is very difficult to grow!
If you have lavender growing successfully in your flowerbed... I would suggest you might have a soil in that flowerbed that is well draining, on the sandy side, or perhaps gravely, and quite possibly leaning toward alkaline. Since those are the conditions lavender likes.
This type of soil is NOT desirable for many of the most popular garden bed cultivated perennials & annuals, and stuff like rose bushes. I believe most plants tend to like neutral pH, or in the case of some shrub plants, liking actually acidic soil best.
Just how well do the lavender do in that flowerbed? Did they overwinter there well?
Unless you want to research the plants you want to plant, and find ways to amend the soil to their liking, and cater to their water needs (with watering and mulching)... you might want to instead find other plants that like the soil... by deciding what HAS grown successfully, and looking up what types of soil & conditions they like... and then choosing other plants & shrubs that also like those conditions.
Or go with the stuff that can tolerate well a wide range of conditions.
I did read somewhere that creeping phlox can grow in a wide range of soil types.
Nasturtium is reported to thrive in a wide range of soil, including poor soil. And doesn't mind a bit of shade.
Judging by my experience so far, I too would rate this plant "easy".
Also, the nasturtium plant actually has some of the nicest foliage I've ever seen. I love it! It's really fetching & interesting. I'd grow nasturtium even if it didn't bloom at all!
Drawback: if you have rabbits... it might not last! The blooms at least.
I think rabbits ate my nasturtium blooms last year, because I didn't see very many, and they disappeared pretty quick so I was never able to harvest seeds. But again, I didn't really mind because I really just like it for the leaves anyhow.
Cosmos seemed to grow good for me in some troublesome sunny areas.
And I read that they do better in "poor soil".
They also don't mind well draining sandy or alkaline soil.
But though the flowers are very pretty, the foliage & stems are rather wispy... and so if you were to plant them in the flower bed you have pictured, I would recommend planting them behind something else, so you have some bloomage & foliage lower to the ground as well.
That's what I'm doing this year in the proper flower beds I want them in. Planting something shorter & bushier in front of them.
I also read that they could need staking, but I have no idea what conditions that could be under Mine were 2-1/2 feet tall & stood up to really high sustained winds we get here coming off the adjacent field.
Oh, and the clematis suggestion is an excellent one!
They too like well draining soil, and a bit on the alkaline side. So they would probably do well where lavender does well.
They can also handle some shade. (Mine is in shade after 1pm & for the rest of the day, only gets morning sun.)
This is my clematis last year:
Bearing in mind this is several years establishment... with the previous occupants here not really having pruned it much over 2 years.