scaffolding and a ladder as you are doing is about the only way I know. Make sure the ladder hooks well down the other side so it don't bounce loose. Use a safety line also.
You could try to rig up a long pipe for an extension on a sprayer to reach as far as you can. Never tried it but a long extension tube on a airless sprayer should do it.
You still have to do your prep work and you might make a sanding block to fit your roof and use a pole with it. Same for a power washer if you power wash it. Good luck.
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