Nutrients in compost will only last a few weeks, so yeah you'll need to feed annuals once a week, once they start flowering.
Dahlias are our go to plants for cutting, as they'll keep producing from now until the first frost. They will store over winter, but to we've struggled with that.
Sweet peas are great in containers, if you create a cane wigwam to support them. Again, they need feeding a lot. My best sweet peas are grown from seed in September for the next year. That way they get a head start and always seem stronger for it. Forgot last year though.
Ask jy when he's about. He's the expert.