It's a cancer caused by aflatoxins.
Bird peanuts used to be the leftovers, quite literally the rubbish that hit the floor, got wet, mildewed and attacked by bugs. However I think you will find they now have to be top grade human quality, so these aflatoxins are not present in them anymore.
A 1kg bag is actually perfect, it stop you using too many. If you work on one bag per trip or for 48 hours then that prevents the 'addiction' that fishing with too many can create. Obviously, don't forget, soak for a minimum of 24hours and boil for 20minutes.
Peanuts while they are high in Vitamin E, due to, I think, an enzyme inhibitor it stops the nutrition being utilised, so high quantities of peanuts are not good for carp, and many places ban them.
Dried mealworms float, might be some alternative thinking coming around, like putting them in groundbait and wet spod mixes for floater fishing, or even as freebies for surface fishing...
Or maybe grinding into base mixes...
As for the wild bird seed, ยฃ1 a kilo may be slightly more than ยฃ12 for 15 or 20kg from an animal food store, but I have bought bags from the supermarket when out fishing just to top up, or not been able to get to the animal feed shop. The preparation is easy when fishing, I just pour boiling water over them in a bait bucket, put the lid on and they are ready 24hours later. You can boil them after 24hours, it releases sugars, and does make them more attractive.