In your opinion, how more viable Ganondorf would have been if he kept this custom moveset from Smash 4 to Ultimate?
His only custom moves that really stood out for me were Dark Fists for up special and Wizard's Dropkick for down special. It would've helped if he still had them as an option, but what didn't help was either the strength or hitbox of some of his basic moves being nerfed from Smash 4 to Ultimate. Down tilt comes to mind for both, though still one of his best few moves in the neutral. The hitbox of back aerial being smaller than before is kinda weird too. His regular up and down specials, Dark Dive and Wizard's Foot, were also made harder to land which further worsened Ganondorf's blindspot issues. Said specials were made stronger, but the nerfs more or less counteract the buffs. Perhaps most of all though was the nerfs to his up aerial, where the sourspot/back hit no longer hits underneath him when used as a rising aerial, thus nerfing most of its gimping effectiveness. Then there's Ganoncide now being escapable by mashing depending on his % and the opponent's and kills him first now even if it does fully land; I know they did this for all the villain command grabs, but it hurt Ganondorf most of all with it previously being one of his few reliable kill options.
Do the other buffs he got (the new sword smash attack and such) make up for those nerfs? Debatable. I'd still argue Ultimate Ganondorf is probably the best Ganon we've had overall (though I don't remember Melee 'Dorf much at this point), but his weaknesses are even more exploitable, making him more polarized than ever. He did not need all those nerfs, unless the intention was to keep him the #1 worst character in the game, which he still is. It was debated for awhile between him, Dr Mario, and Little Mac as to who the worst was, but when the latter two got more buffs during one of the last balance updates, it became inarguable again as to Ganondorf being rock bottom on the tier list, despite being mostly better and in a better spot than he was in previous games, especially Brawl.
No matter what kind of buffs he gets (significant ones that he got from Sm4sh to Ultimate admittedly), he's forever cursed to be a character Nintendo's afraid of ever making that good because of the balancing between competitive and casual, and in casual settings, he's
the character to play; someone has to play the noob killer role I guess, and Ganondorf has always fit that to a T.