The build has some definite strengths, but a "demigod" with an AC of 13 (12 without a Simulacrum's help), zero bonus to Wisdom or Intelligence saving throws, and no viable capacity to see an invisible enemy is not ready for a solo "against the world" free for all.
Against any level 20 character with their primary attack stat maxed they will hit you on anything but a natural 1, unless it is a Sharpshooter or Great Weapon Master situation. Having over 200 hit points and resistance to all damage is still not going to cut it against an optimized character who hits 95% of the time.
Against any level 20 caster with their stats maxed this guy will only succeed on a Wisdom save (or the much rarer Intelligence save) on a natural 19 or 20, so 10% of the time. That opens up a lot of weakness. Against a level 20 Lore or Eloquence Bard who can burn inspiration for debuffing his chance of success ON A NATURAL 20 is 1 in 12, with no chance on a natural 19. This means that even spells with repeat saves like Slow or Hold Person are likely to last a fair while against him (and his Simulacrum) provided the caster maintains concentration. It also means he's going to get the benefit of only every 10th long rest once a hostile caster with the Dream spell learns who he is.
You would do well to change the Contingency trigger to be more specific. Lots of spells and effects that would trigger "I can't cast spells" would be unaffected by Greater Restoration.
I suggest you to create/show a build that could effectively challenge the DEMIGOD.
Unfortunately, nothing you posted would work against him.
He has many defenses, it is difficult to beat him.
1) The best defende of all.
First and the strongest defense of the sorcerer is deception and disguise. It has effectively a improved mirror image with more than 60 clones that is disguised magically and mundane. Subtle Spells allows him to cast spells freely without reveal himself.
It also can be everyone, including your ally. +17 Deception check, Actor feat, Disguise kit, Magical Guidance, Extended Seeming spell is pretty hard to deal with it.
2) The Demigod real HP is 714. He doesn't fear creatures damage. If Sharpshooter and Greater Master Weapon one attacks the sorcerer, the sorcerer deals more than 260 damage at 660ft.
If close enough, It's more than 1.000 damage per turn with his minions. The bard can't survive more than a turn.
3) The bard will need fly and cast a spell against him, but the sorcerer blasts at 660fts and can fly. All spells mentioned can be counterspelled.
Also, It's pretty hard to keep concentration dealing with more than 1000 damage per turn.