https://www.amazon.co.uk/2018-PROGUTTER-Gutter-Cleaning-Scrapers/dp/B078YDMMWG/ref=sr_1_6?crid=KKVYYF82HI2U&keywords=pro+gutter+cleaning+tools&qid=1666637517&qu=eyJxc2MiOiI0LjIzIiwicXNhIjoiMy4zMSIsInFzcCI6IjIuOTAifQ%3D%3D&sprefix=pro+gutt%2Caps%2C113&sr=8-6
That with an extension pole will do...
Your slates are fine, they are just a cheaper quality slate. The rust will happen now, nothing you can do except replace them.
The lead is an issue, whatever they have used is reacting with it badly.