Time is now limited for second sowings but if you have plants there are more options. Check out the banner "Second cropping" under Useful Advice.
Spring cabbage want sowing asap, for planting by end September. If you have plants now, they may heart up before winter, depending on weather. They grow better from being sown in modules or a small row, then transplanted, Spring onion and spinach can be sown direct. So can salads such as lambs lettuce, rocket, mustards (they are one of many oriental leaves) and endives, also winter varieties of lettuce to grow in spring after establishing now. Broad beans don’t want sowing until November.
Rotation is not critical, if you are strict you can either count a second sowing as part of the same year (eg rocket after psb) or grow plants of a different family (eg lambs lettuce after psb).
As for compost, like so many answers I give, it all depends…. did you use much to make beds with, and is your soil in good heart? If you spread 4-6 inches in spring, no need for any more now. If an inch or two on average soil, the same again is worthwhile now. Once you do that for two or three years, the summer composting is not needed.