Reducing map size reduce the impact of the snowball (if the map is small the good thing is, when you reach the snowball point, you've kinda almost win so you don't have to spend 10-15 turn to finish things)I think a strategy game like this is very snowbally, where the first few turns are the most crucial to succes.
Not mind controlling the best enemy target each turn puts you at a much weaker position the next turn compared to players that do the most optimal opening.
In the later Hard mode maps, I look at how enemy behaves and look for openings, plan out what I want to mind control for the first few turns. If I can find the optimal opening, the rest of the map becomes trivial once you start building an insurmountable opening advantage like that.
If we get overwhelmed by the enemy, it usually meant the opening wasn't strong enough to capitalize on their options and the enemy start to build up their own insurmountable advantage that can get out of hand.
Chapter 14 Hard, I had to really think how to build the best possible opening as your immediately get attacked by 2 knights + 1 Fencer very early and they start to build more and more knights each turn. You must kill/mind control more knights than the enemy produce each turn or there will be more and more knights than you can handle over time.
Not sure what the best way is to make maps easier, you could lower the enemy income/bases for example to lower how much the enemy snowballs. If a player overcome the first 3ish turns with the same map layout as normal/hard mode, but their own army is in shambles afterwards, it should ease the pressure as the enemy can't build up their reinforcements as fast to capitalize on the player's much weaker army than players using the best possible opening moves on that map.
Reducing enemy income is probably a good idea sometimes, or maybe, having more space between enemy bases, so when you managed to capture the first one, the enemy ability to flood you is hindered so you spend more time fighting waves on enemy when you know you already won.