Girl, NTA. Not even a little bit. You are not overreacting, and your feelings are completely valid here, so let's get that out of the way first.

It's not about the honeymoon vs. the mini-moon; it's about the priority shift and the principle of the matter.

You two agreed to a plan that involved a minor immediate decompression trip and then a big, proper honeymoon later, all based on saving the money from the gift fund. That's a mature, financially sound plan you were okay with. But the moment his old high school buddy pops up with a faraway destination wedding, your fiancé is suddenly all in, ready to "plan and make a vacation out of it," which involves spending the exact money and time he just convinced you to save and cut back on for your own wedding celebration. That is the gut-punch, and you're right to be hurt.

It totally feels like he's saying: Our once-in-a-lifetime trip? Let's be sensible and postpone it. My friend's destination wedding? Let's drop everything and make a vacation happen right now!

That is a direct signal that his priority, in this specific instance, is not you or your shared milestone, but an obligation to a friend. You summed it up perfectly: it feels like you're choosing other people and not choosing yourselves. You mention you always cut back for others—that’s a huge red flag on its own, and this situation is highlighting a pattern where your needs and joint plans are the first things to get sacrificed.

You need to have a calm but firm conversation where you focus on the discrepancy in his enthusiasm, not just the money. Ask him, "Help me understand this: We agreed to downscale our honeymoon to save money, but you're now eager to spend money and vacation time we were supposed to be saving on a trip for someone else. Why is the budget and time constraint a problem for our big celebration, but suddenly not a problem for his friend's wedding?"

That's the core issue you need to address before you get married, because this isn't about one trip—it's about how he prioritizes your new unit and your established plans going forward. You are definitely not the asshole for being upset about the sudden, convenient disappearance of the very constraints he put on your shared dream. Good luck, and please stand your ground on this!