Let's get started...

I'll analyze the creatures...

Goblins, Elder Goblins.
We won't get much use out of Goblins. Only Elder Goblins are useful because they can repair golems.
Conclusion: upgrade to Elder Goblins and use them to repair golems.

Gargoyles, Obsidian Gargoyles.
They aren't very useful. They mainly serve as a shield against ranged attacks.
Conclusion: no need to upgrade them; it's better to spend the money on something more useful.

Iron Golems, Steel Golems.
They are slow, that's true. But they are strong and durable enough for their level 3.
Conclusion: upgrade them. Not urgently, first get Mages, and then focus on upgrading.

Mages, Archmages. Their attacks go through everyone on the battlefield, and Magic Fist is also good. And if you upgrade them... Fireball... Archmage is worth the cost and fully pays for itself.
Conclusion: upgrade them. Definitely.

Djinn, Sultan of Djinn.
At first, they don't seem very strong, but with the upgrade, new abilities appear. Djinns are ambiguous in all aspects; they seem strong, but what can they do? Magic... and that's it... They aren't very strong in combat.
Conclusion: you can and should upgrade them. But sometimes Djinns aren't very necessary. There are cases where it's better to quickly deploy Rakshasa Princesses instead of messing around with Djinns.

Rakshasa Princess, Rakshasa Raja.
Many advantages and practically no disadvantages. It's almost too boring to describe them. All positives. Conclusion: upgrade them.

Colossus, Titan.
You need to upgrade the Colossus as quickly as possible because only the Titan can bring enormous benefits with its shots. The Colossus... doesn't shoot... Conclusion: upgrade it.

If anything, correct me...