You can add a slug: firstpost to your posts frontmatter and then posts: "/:slug" in the global configuration.
Cutting off the first x characters in a config file is impossible.
Another approach would be to rethink the filenames. If those numbers are there to sort the items on your website have a look at the frontmatter variable weight. You could move that into frontmatter and sort by weight. Then of course you will loose the sorting of the files in your filesystem too, so that might not be the preferred outcome.
Those numbers are a quick reference for me to know the date I created these posts in without having to open every file one by one.
This is unfortunate. It would be awesome if one could define his own slug function. Replacing with slug almost solves the issue since it reads “title” property and adds hyphens to it. But if title is different than filename, it must be done manually…