Indeed, there's no reason you couldn't just add "_" and "-" or "." as well to complete the set. Your identifier will still be URL-safe. I've been using this type of encoding for years [1] for these kinds of ids to use in URLs, and encoding/decoding is super-fast with some bit shifts. And unlike Base64, you don't need padding characters.
[1] https://sourceforge.net/p/sasa/code/ci/default/tree/Sasa.Web...