Strength and dexterity work well as it is.

Intelligence, as the 'magic' stat is underpowered. Most spells use intelligence as max damage, so even if you raise it constantly it will become useless fast as  30 damage even by early midgame means nothing.

Change: make it the same as STR and DEX. Give attack spells basic damage. Have damage scale up with your researched magic level and intelligence as modifier.

Example: casting Firebolt with 15 int (+50% power)

Firebolt: base damage 10

Damage: 10*(1+(15-10)/10)=1d15 damage

For summoning spells level up summons based on intelligence modifier.. 1 level per 4 int sounds ok.

Every other spell you can lower the mana casting cost.

If you want to rewamp the magic system further, give a new attack spell for every level of research.. more mana for more damage but better mana/damage ratio overall

Wisdom: please detach mana(essence) from wisdom. Give heroes a base of 5 mana per level * wisdom (+10% per point above 10) modifier. At level 10 that would mean a mage specialised character could have in the area of (50-100) mana..

You can also use int+wisdom as base for magical defence.. checking magic vs armor when armor goes into 100s is silly. Something like (int+wis)/2*level sounds ok.

Constitution.. same reasoning. Give heroes 10hp per level*modifier (at level 10 that means heroes with 100-200hp.. which is not too much considered you have monsters that can hit 100+)

Also have the imbue champion start a champion with 10 wisdom and cost the sovereign 3 of his..

Put strenght/dexterity requirements on hero equipment.. sorry but seeing mages in full plate just makes me sad. You can then add int requirements on 'magical' pieces of equipment so everybody can't use everything under the sun.