The filenames contain information that you need in some way for the scheme to work.

You are combining different parts and inserting a missing byte every time you combine the files. You need to combine the parts in the correct order, and the order is part of the information that makes this work.

If the ordering isn't coming from filenames, it needs to come from somewhere else.

You could do the same spitting trick but only split at progressively increasing file lengths at the character '5'. The "compression" would be worse, so you'd need a larger starting file, but you could still satisfy the requirements this way and be independent of the filenames. The decompressor would just sort the files by increasing length before merging.