Have you considered some terracing, perhaps using old railway sleepers or dry-stone walling? the flat terraces slow down and catch the run-off of rainwater allowing the water to filter down to the roots. They also make it easier to plant beds.
Our local electricity company last year renewed some poles and (kindly?) left the old one lying on my land. They’ll be cut to length and used as step fronts or terrace walls (2 deep) this year! it’s an ill wind…!
There’s lots of useful stuff on YouTube about making & using charcoal. Leave the coal-ash to your local council!