Keaton did a short career retrospective on GQ's YouTube channel, and spoke about his experience and approach to playing the titular character. It's the time I've heard of Keaton mentioning Frank Miller's name as an influence on the film. I guess he must've skimmed through Dark Knight Returns as an idea of what the tone that Burton and co should be aiming for.
https://youtu.be/j2h4lcXUCxw?t=17
If I remember correctly Sam Hamm cited TDKR as something he had read prior to doing the script.