1. No he doesn't need multiple movies of build up but even if he did him showing up and say beating tar out of couple X-men and disappearing would be build up and bigger showdown can happen in other movie
2. His obsession with Summers line makes sense as a geneticist, Summers family represent one half of perfect strain so to speak,It doesn't matter if other lines might do the trick the Summers line will do the trick.
3. His origin being all over the place does not hurt. They don't have to tell it all in one movie Wolverine and Cable have convoluted back stories and neither X1 and Deadpool was hurt by it, you just need to tell what information is relevant to the movie b. A good movie adaption will choose parts of the story that work the best and tweak them, X-2 changed Stryker from Preacher to more of Soldier and that way they were able to sneak Wolverine's Weapon X storyline. I think most fans where fine with changes made because it was well done.
4. If it was Fox with Ivan Ooze maybe you should worried about the looks but MCU was flawless with Thanos beside Sinister is a two part look.
The first part would be ,more practical and probably look like this
The second part could look something like this,when he decides to be serious and be more monstrous
Capture.jpg
Of course this whole lot of defense of a character I probably wouldn't use as the first villain in a X-men movie. My choice would be something like this
MCU X-men 1. Magneto and Acolytes start strong (and remove Magneto from the table after)- Mutant Genesis mixed Ultimate X-men 1
MCU X-men 2. Mr Sinister and Marauders- Mutant Massacre mixed with some other story probably something with Hellfire Club.
MCU X-men 3 part 1 Bastion and Humans(Graydon Creed)/ Legion- Legion Quest with hint of Operation Zero Tolerance
MCU X-men 3 part 2 Apocalypse/ Bastion & Nimrod/Magneto- Age of Apocalypse/ Days of Future Past with a hint of Operation Zero Tolerance.
I am not in the camp of avoid the best villains because of Fox, We have a limited window with actors tell the best stories use the best villains.With Acolytes around you can use Magneto more final boss with Acolytes doing most of the heavy lifting as the villians ,Beside after using Wolverine and Magneto in the first movie. I would send Wolverine off to do solo projects until part 3.Would fake kill Magneto and bring him back as antihero in movie 3 part 2. After successful run with classic stories you break out the different villains.
MCU X-men 4. Ord(or any other villain)- The Gifted
MCU X-men 5. Cassandra Nova- E is for Extinction/Torn
MCU X-men 6. Messiah Complex
MCU X-men 6 part 2 Messiah War
Something like that is what I would see