Also have very heavy/clay soil Woof, and I think I have tried all the suggestions, except seaweed.
They all helped. The one thing not mentioned, that I noticed, was PH. It's very alkaline here, and
choosing amendments that helped bring down the PH helped a lot. Things like pine needles, leaves off
the oak trees etc. The good thing about the soil, being high in minerals, is that when you get it to
the usable state, the veggies all have awesome flavour.
Good luck with it.
Almost forgot, I have a lot of cast iron filings developed in the shop, I clean them up with a magnet,
and spread them around lightly. Not sure about the garden, but the trees really like it, and I seem to
get more earthworms.