I don't grow Sea Thrift, so can't offer any advice there.

In terms of the Astilbes, I have lots and grow more from seed. Keep them away from midday sun - most of mine get morning sun with dappled shade the rest of the day. For the first year or two, make sure the soil never dries out around the roots. After they are established, you still have to keep them in moisture retentive soil, but it's not so much a worry.

I feed mine some 'Organic Better Than Bonemeal' from the Superstore (Loblaw's) and give them a good mulching of organic matter (I use compost which I make).

Here's some grown from seed in their thir year.

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I grow both. Astilbes do best in part shade conditions. It does okay in full sun but when compared to those planted in shadier areas - the shady area ones are healthier. As for fertilizers - Since these are undergrowth plants, all they need is compost and some organic fertilizers containing trace minerals - like bonemeal or blood meals.

As for thrifts. -- they can grow shallow rooted in sunny areas. I like to space them in between rocky areas. For soil I like to use triple mixes only. You can deadhead them if you wish or you can simply leave them be. Deadhead them close to the base - simply because it looks better. They will continue to produce more blooms - I believe summer long.