he is leaving it a bit late to do so because of his age, alot of people tend to forget his decorated amateur career
Better, IMO. Only time, and the challenges he chooses to accept will tell. I will say the current MW division isn't enough to get him over that hump. May need to seek something in other weightclasses
He is Not the Undisputed MW champion. Undisputed means, no one else can hold that claim. GGG is not even THE Champ, He is not the Lineal Titleholder. That belongs to Cotto at the moment
Better than X will always be subject to debate but if he beats the winners of Canelo/Cotto & Lee/BJS, and retire undefeated after breaking Bhop's record with 4 more decent title defence, he's an ATG and one of the best career middleweights of all times. Exactly where you place him the ranking will always be subject to debate.
Hagler was beaten in his career and drew with Viti A.Regardless of resume[within reason]the favor has to go with the unbeaten fighter especially one that's scored a k.o in every world title fight he's had.
So Wilder must be the goat HW. You say within reason like GGG has fought 1 elite name. He's fought one guy that held a world title, and his best opposition is solid at best.