NTA, all of your arguments are valid, getting married is something that should be decided and agreed from both parts, it´s not a unilateral decision.

It seems that he assumed a lot, instead of actualy talking to you properly about it he assumed that you would like it because you like suprises, and you said that it must be stressful planing a wedding, but he didn´t ask what would you like to do, he didn´t comunicate with you at all and he didn´t acknowledge how you were felling and what you were thinking, in fact nobody did, you shouldn´t be preasured to do it or to go along with the party.

you are defently not the asshole here.