I'm agree with you about the plodding, the anime could easily jump from episode 12 to the last 3 episodes, however the "filler" episodes are still fun to watch even though they don't contribute too much to the main plot.
I feel the ending as a bittersweet experience but also a necessary one for both Hyakkimaru and Dororo. I would like to see something else about this anime but considering there's a big chance to mess the original experience, it better keeps this way.