Anubis_Creationz
For those that think option (B) is unrealistic or hard to write about. It's not that unrealistic. He'll start slow by making alliances with, inhuman settlement on the moon, the savage land, the inhuman settlement on earth, the Brotherhood, the mutants, wakanda and the Asgardians. Possibly Nova prime, and possibly even recruit some of the dark elves. He could make soldiers using cloning technology acquired from Nathanial essex. Use Limbo's time dilation to his advantage. (He could leave them to train there for a few years.) He could mine Vibranium from the savage lands and use that vibranium to buy space ships. I have a bunch of other ideas, but the point is it is doable. How do you think Thanos put his army together ? He conquered planets, recruited soldiers, stole ships, bought ships, and even hired mercenaries. Cypher could do the same, he may even stumble on a planet from which, he could recruit soldiers, or buys spaceships. P. S ( I'll never have him stop work on his personal strength, he'll eventually reach the level were he has to battle very powerful cosmic beings. )
I choose D. My reason, he already has organization and connected close to Kamar-Taj by learning magic from Kamar-Taj and promise to protect the Earth, so the MC just prepare for Thanos incoming party by doing filler or doing Earth super Villain battle before times up (killing Thanos). Need enemies? Earth already has many super Villain whom has very broken abilities, power, magic, mutants, inhumans, enemies from hell, enemies who come from parallel dimension, enemies from another dimension, etc... and he doesn't need to add more enemy from outer space if the MC want to life free from tor *chough* everyday battle, every time battles, and everywhere troubles or making him having being chuck-holding his love life from the trouble, battle, world destroyer, multiverse destroyer, and Dimensional destroyer.
I think similar to a lot of commenters that some combination would be best (I am actually thinking you would do something like that based on how the story has gone so far honestly.). A. I think teaming up with the avengers is better than straight up joining them. Similar to how Captain marvel was at first where they could call her for help, but she may be out doing her own thing and unable to respond. B. Cypher should develop some sort of space presence since there is so much in marvel that goes on out there in the wide universe which could greatly increase your writing options. C. I mean people like OP characters, but just this alone would not make be an interesting and would be a little different than what has been happening in the story. I do want him to keep increasing his power because that is part of the fun of making strong characters. D. This option goes well with what Cypher has been doing for awhile now in my opinion. He has started building a team already and trying to make connections with people and groups so that he will have more allies. This option just takes it further, which I think is a good thing since Thanos does have an army which Cypher will need to counter. I like those huge fight scenes more than just one person just soloing the boss (those are good in moderation, but I just like big fights with more people like when everyone went into Limbo). Those were just some of my thoughts, but like you said at the end, you do what you want. I fully support that since I find the story to be great so your choices have been on point, and I trust you on keeping up the good work.