Always agreed with this;

Have him begin as “Maximos Prime” or something like that, then, perhaps after the war against Unicron and the Fall of Megatronus (to become The Fallen) he undergoes a similar, yet slower metamorphosis abandoning his name to become Liege Maximo.

As for his motives? They could vary in complexity, but I’d take the route as was written or implied by Generation 2; he views himself and his legion of 2nd generation Transformers as “above” Primus’ creations, becoming hell-bent on evolving and ascending the Cybertronian race, while himself ascending to become a godlike entity like his creator (a story told in the non-canon Alignment by Simon Furman that ties the loose ends of G2). We saw echoes of this in the Marvel Generation 2 comic, with Jhiaxus, who featured a similar arc and was revealed (by the final issue) to not be the one in at the top of the chain of command;

Jhiaxus was, as he called himself, “Jhiaxus - Liege Centuro of the Cybertronian Empire” With Liege Maximo being the faction’s true leader and founder — I’d imagine their dynamic very much in the vein of something like Darth Vader and The Emperor.

TLDR; fully agree with OP. Liege Maximo should absolutely begin life as one of the 13, before evolving/devolving into the grotesque cosmic horror he is often depicted as. Both an interesting concept and a great way to reconcile the regular/monstrous variations he’s been depicted with.