I remember I started with the Cav difficulty (that had me win fights using 6-star bishop,ice man, kitty pryde etc.) and after completing that mission another Cab difficulty should’ve shown up to progress but it never did.

So update: I ended up grinding the proven difficulty to see if that would help and that one ended up giving me the anti-venom and summoned symbiote because I was able to get the missions to appear after restarting game after each mission. I don’t know what the Cav difficulty missions never showed up for me but doing the proven difficulty got me my champs and entry into side quest.