IMO he's pretty good early game too when you can equip your starting units with items that have 2 layers of enchantments.

Heres what usually do. Take at least 4 destruction spellbooks. Start with the ring/amulet that give +25% experience. This item will arguably be useful the entire game. For this reason you can use some of your powerful 1-time use enchantments you get from spell books without feeling too much like you are wasting them on weak early game items.

Take this item and enchant it with +3 cold damage and +3 fire damage. Boom, you have a really powerful starting item that greatly boosts your first hero.

You also start with a +3 normal damage enchantment. If you dont mind using it early game(I dont because later on you can get +2 damage from platinum ore), put that on your first hunter's short sword. If you start with a hero that cant equip swords, save and reload and now they can equip any weapon you enchanted before saving. I dont consider this an exploit because many units that cannot equip swords have few/no alternatives.

Those are just a few example, you have other things like one +3 defense that you can use to make some decent starting armor. I prefer to use some of these powerful enchantments early rather than saving them for the perfect opportunity because they have more of an impact the earlier you use them and the game has an inverted difficult curve. Sure there is a tradeoff, but starting out strong means more experience, more lairs cleared, more quests complete, more cities founded, etc that will help you snowball. Plus - as you mentioned - the tinkerer is really strong late game either way.

Besides that you can also use things like +1 tactical hp regen, +poison damage, etc to make some "throwaway" items that greatly boost your power early game.

Also on a side note, you can balance out his spell casting limit with fireball/blizzard scrolls and Raza the necromancer or Keplin the mage.