Aging a conlang into separate languages?

Hey all, this is my first clang and I've been chipping away for about a month. I started with the "modern" version of my language, developing out all the rules etc, and now that's relatively finalised I decided I want to make the Classical version and create 3-4 steps of changes to age the language (important for world building).

The issue is that although I have the changes necessary to make it fit the phonotactics for the modern version, I kind of want to evolve the classical into a couple of extra languages/"dialects" and I'm not sure how to start. I know it would require different changes and divergence at different periods, but are there any ways to really decide or find which might be the most realistic?