I've given two of these after having been asked to do so during the reception (twice!). So here's some advice from an ad-libber, no bromance required:

Tell a story about you and your friend. Maybe it's how you met, or an event that showed you that they were the kind of person you'd do this sort of thing for - either way, say something about this characteristic of your friend will help support their relationship with their partner. Try to fit in a joke about an annoying aspect of this positive trait that friends or family might have made in the past.

If you know the spouse well enough, tell a short story about them and why they're a good partner for your friend. Drop a joke about how they'll (finally) be the sensible one who will reign in the annoying aspect you joked about earlier.

Say something about how great they are as a team, and express a hope you have for the two of them as they start this new aspect of their lives. Tie it back to how they deserve great things because of the things you said earlier. If you wouldn't be the person standing there giving that speech without your friend, say so - regardless of whether you decide to speak about being trans during this speech or not.

Remember that laughter is what happens when expectations are subverted in a surprising way, that everyone loves a good callback, and that groupings of three are more impactful, and you'll do great.