The entity created by Robert E. Howard (of CONAN THE BARBARIAN fame), Shuma-Gorath, is one big, nasty piece of work. A Cthulhu-esque interdimensional creepazoid thingy obviously based on Cthulhu, immensely powerful, more or a less a god. No way, then, would Shuma-Gorath be *anybody’s* minion, no matter how powerful that person might be. In DOCTOR STRANGE IN THE MULTIVERSE OF MADNESS, Wanda Maximoff is incredibly powerful, as much as Doctor Strange if not moreso. And she’s using the Darkhold, which makes her even more of a supernatural powerhouse. But still not powerful to be commanding Shuma-Gorath.
Probably this was the secondary reason why they changed the name of Shuma-Gorath for the movie. (The first and most important reason was to avoid any potential copyright issues.) So Shuma-Gorath became “Gargantos”, which is actually the name of a different critter from the comics, but a character that Marvel (and by extension, Disney) owns outright. Since the creature *looks* exactly like Shuma-Gorath (from the comics) but isn’t called that, I postulate that it in fact *isn’t* Shuma-Gorath. Who’s to say that there can’t be an entire family, or an entire race, of cyclopean, tentacled terrors out there in the multiverse? Shuma-Gorath, then, could be Gargantos’s big brother. Like, maybe a few hundred times bigger.
Had Shuma-Gorath showed up in the movie instead of Gargantos, Doctor Strange would have had to conjure Godzilla (another character Marvel has licensed in the past but does not own) to deal with it. That would’ve been amazing, but would also have resulted in a much longer movie. They would’ve had to split it up into two parts like they did when he Avengers fought Thanos. (Now that I’m thinking about it, I’m really disappointed they didn’t go this route.)