Hello SiW !! Must say, you're very inventive (love the 'slinky' idea
) and persistent. You certainly do deserve success.
The only thing I can suggest for the crow problem is a ground feeder cage, which prevents anything but small birds - blackbirds soon learn to use it. Your crow problem is my feral pigeon problem and a cage works for them. The use of a ground feeder cage is better for birds like, say, chaffinches, who can't hang onto feeders. My only feeder now is an anti-squirrel feeder which dispenses peanuts.
Re the black sunflower seeds, presumably you mean the ones with the husk still on ? The huskless ones make no mess at all and are very popular with all birds.
I have, sadly, discontinued mealworms, it's just not worth it. I now stick to suet pellets (insect), mixed wheatless !!
seeds, and huskless sunflower hearts.
Great to hear from you again