Balancing a 1 player vs many AI game

First let me put you in context. I have 2 game ideas that are similar with each other.

A- A parody of star wars where a player is the dark lord and controls an empire that wants to control the galaxy.
B- A sorcerer who suddenly decides to declare war to everybody and want to conquer all adjacent kingdoms.

Both would have a board similar to catan or Twilight imperium where the player starts in the middle and the AI starts in the 6 corners or 6 sides. The AI would spawn units and then they will move according to a path, like in any tower defense game, toward the player's home.

For the space game, each hex will give resources that the dark lord(player) could use to produce ships to attack their opponent. To counter effect the fact that bigger player empire means more power, rebels are going to randomly spawn and attack the player's territories. There are 2 pawns, one for the dark lord and one for the apprentice. The player can play power cards when a battle take place where the pawns are located.

For the sorcerer game, it's slightly different. The troops would follow a path, but the terrain is much more important. Sometimes a bridge would only allow 1 or 2 unit to cross, forcing other units to use another path. Some water area requires a ship allowing moving troops once every 2 turns. The territories gives no resources to the player and he must summon all his creatures with spells. He cannot build troops, but there could be an option to build troops in captured kingdom. So magic here is much more powerful and can be cast anywhere. There is also no rebellions.

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Here are the suggestions I have received so far.

Redistribution of power : Consist in making other AI more powerful as AI falls. The problem is that it creates the hydra effect where killing people make the other stronger. A suggestion was to transfer weaker bonus. I don't really like the idea. It also create a situation where the player do not want to kill the AI to prevent a difficulty raise.

Diminishing resources with time for the player: I could me another solution but I really don't like it for my game because I want to give the players the feeling of power.

Make Players end far away: This is the idea that an army pushing and destroying an AI will get stuck far away from other kingdoms. This concept will we implemented in the game and will be more important near the end especially if the AI are far from each other.

AI gets added as the game progress: This is the most promising idea. Either an artificial diplomacy makes AI enter later in the game. Either some AI are not ready to attack and needs to build up. Either some Ai have difficult terrain which slow them down and takes much more time to invade.

Strength of the AI increase with time: This is something I could do. My idea of making AI come in what somewhat that idea, but if I want all AI in, I could make them progress with time, like if there were now more prepared or technologically advanced. (This is what starcraft solitaire does). I will use similar mechanics for the rebel attacks. The more territory you control, more locations will be attacked by rebels.

Thanks again for the input.

I think I would want the player to eventually face 6 AI, and that the goal of the game would be to destroy all the AI.

Still, there needs to be some sort of progression to make sure the game will end. So if the player stall too much, the AI must become stronger to terminate the player.

I have not considered what will happen when only 1 or 2 AI are left, since there could be less challenge and it could become obvious that the player will win. The rebels will sure give a twist and create disturbance, but I will have to find something else for the other game.