Writing a regular grammar for a regex of decimals
Regular grammars (of the right-regular variety) are essentially equivalent to NFAs, in the sense that they include roughly the same information. Once you convert your regular expression to an NFA, you can easily construct from it a right-regular grammar.

What doesn't seem possible is adapting your logic to construct a regular grammar. Unfortunately, there is no simple way to convert an arbitrary context-free grammar to a regular one; indeed, it is undecidable to determine whether a given context-free grammar corresponds to a regular language.