Upgrade from gold to gamepass ultimate is 12.99 a month?

That's correct. Upgrading to Ultimate just involves purchasing Ultimate on top of what you already have. You don't get a discount if you're upgrading.

The button on my Microsoft subscriptions page shows 'Upgrade for 1 euro'

Also wondering why there wasn't a warning, just spent 200 Euro to have to spend another 600 on the exact same product lol

Have you previously had Ultimate? The 1 euro promotion is for new subscribers.

The system does show you how much you're going to pay before you complete the transaction.

when you upgrade to gamepass ultimate from gold it clearly states this will convert your remaining gold into gamepass ultimate.

when you upgraded to gamepass ultimate it will have converted your gold to gamepass ultimate, you would have had months of ultimate from the result but you cancelled which cancels the lot, it has it in big red writing when upgrading that it will convert your remaining gold to game pass ultimate. reading before purchasing actually benefits the individual. they have not stolen anything.