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Yes, I know you're already tired of suggestions regarding secondary skills, but I can't help myself. I'll write down a few ideas that seemed most interesting to me:

1) First Aid Tent

On Expert difficulty, the tent provides "area" healing, restoring 100% health to all units in a given area. If the tent is destroyed, it is restored after the battle.

2) Eagle Eye

The ability to view the opponent's spellbook before the start of the battle. On Expert difficulty, you can immediately learn one spell from it. Before the battle, the enemy receives a notification: "the opponent is viewing your spellbook" or "the opponent has learned a spell from your book."

You can create a similar function on the adventure map so that on Expert difficulty, you can steal a control spell.

3) Training

In Might and Magic, this skill gave a 100% bonus to training on Expert difficulty, and in the end, it could be forgotten and replaced with another skill. And despite all this, it was still rarely taken, so I suggest adding the following functionality in addition to the training bonus:

When leveling up, you are offered a choice of 3 skills (instead of 2) for basic training, 4 for advanced training, and 5 for Expert training.

There's also one more thing that comes to mind (now pay attention, no one has suggested this to you yet):

The ability to refuse a skill in the witch's hut and with the scholar!

4) Diplomacy

Leave the percentages for joining (possibly only half the stack), but now on Expert diplomacy, neutral troops always offer to let the hero pass without a fight for a certain fee. That is, they always seem to run away, but they don't run away, they just let you pass for money.

That is, now you can, for example, quickly level up diplomacy, collect money, and go to the center on Jebus in the first few days, receiving a certain bonus.