It's never too soon to start thinking about succession planning, even before the Imperium is established ...
We do not know that the Murphy succession is governed by primogeniture. What if, instead, the Murphy inheritance was governed by the Gaelic practice of tanistry, where the heir is chosen from among the (extended) family by election? Who would the electors be? Probably not the 500, at least not as a corporate body. Fleet commanders? Would that ensure the selection of Callum, or of Vyom?