As someone who has used Specs Mag for the entirety of SM, I think his original move-set is far superior.I think you'd be better off running Fleur Cannon/Bolt Beam/Focus Blast, because:
Fleur Cannon is a stab
Tbolt 2HKO'es Toxapex
Ice beam damages Zygarde and Lando-T without reducing your SpA
Focus Blast hits Heatran
Not much switches into a set with these moves..
Also, Flash Cannon is not needed when Fleur Cannon hits Fairies hard already, unless you want to cteam CM Clefable but who cares about that mon anyways lol
There is zero reason to use Ice Beam on Specs Mag. Fleur Cannon KOs Zygarde and Lando-T as you said, and it's a far more spammable move than Ice beam. The SpA reduction is irrelevant because if you kill one of those Ground mons, the job is already done, and you will almost certainly have to switch out after anyway (are you ever going to sweep with a +1 Magearna locked into Ice Beam?)
TBolt is also useless. Specs Volt Switch is a super useful pivot move that basically does the same thing as Tbolt but it eases prediction a bit. 2HKOing Toxapex is irrelevant because as soon as you hit him once, he'll know you are specs and can just switch out into a ground type. At least Volt Switch is useful when you don't have the guts to predict the Heatran switch-in but don't want to lose momentum.
Flash Cannon is surprising useful because it's still a strong STAB that is more reliable. It has a lot of random but useful uses against stuff like CM Clef (which is super scary btw), Mimikyu, Gengar etc. It's far more useful in practice than it seems on paper, and it's definitely more useful than Ice Beam which I don't think would see any use in an actual game.