Finally it seems Google Maps allows choosing a custom route path, rather than merely offering a couple alternative Routes. It uses the [Via method] however it works from the [Thin (or "Vegan") Hambuger] verticle dot menu on the Upper right after choosing an inital destination.
Although its confusing. You pick [Add stop] which adds a Next destination option to the current destination. Then a [Pop-out] menu in-between the [Your Location] and [Final destination]. Basically click the 'unlabled' [Hollow] circle and this expands into a list of Editable intermediate destinations.
Its far from perfect.
First its hard to use until you know how to use it, and the documentation (such that it is) is frankly terrible... they should hire a real Writing Editor (as in a real living breathing 'Person'.. not auto spell check).
Second the ETA and stats on the journey are not updated for the entire journey, but [revert] to only the next destination.. treating this as a series of trips from one Via to the next Via.
It is editable in that you can change the Vias already in a journey.. but they are not draggable on the Map.