Hi Antonia, I think these aphids are reminding you about spacing your plants too close, which stresses them a bit, creates a nice place for moulds too with all the leaves pressing against each other. I would thin plants to recommended spacings, as long season brassicas like space to flex their leaves.
Don’t woryy about diseased leaves on your compost heap, they will convert to something good. There is almost no garden waste you cannot compost, even blighted leaves are fine.