Walter did indeed conduct the 8th quite a bit, including the Viennese premier in 1912. In 1953 he was asked to conduct it in Munich, which he would have loved to do, but failing health and inadequate preparation time made him decline, which is a great loss since we can assume it would have been recorded and we'd have another Walter performance of Mahler to study. His performances of the 8th were always received with "ecstatic reviews", according to Walter's biographers, Ryding and Pechevsky.
Walter conducted all of the symphonies and Das Lied regularly, spending less time with 3, 6, and especially the 7th, which he only did one time in 1920.