Transfem

Echoing the suggestions from others, don't mention being trans or strangeness at being best (wo)man. You don't need to explain yourself or anything, just talk about your friendship with the groom, the happiness you've seen in the couple, talk about things they share that make them such a strong couple, and if you wanna bring some humour you can always share a story that might (lightly!) embarrass the groom but that you can use to maybe highlight a positive quality he has or talk about how he's grown since then.

Your role there is to give the guests some insight from a close friend into the couple's relationship and how special it is, and to help the newlyweds feel the strength of their relationship as they solidify it with their marriage. You can talk about yourself if it genuinely comes up in what you want to say, of course, but only if it's something that's ultimately more about the couple.

The speech should be about what you admire about your friend, and his partner. I wouldn't even mention you being trans unless that's a large part of your friendship.

Introduce yourself, tell a fun story that highlights what a great guy he is, explain how the couple is perfect for each other, and invite everyone to have a wonderful time.

I ran a dj business for about 15 years until covid and the sucky state of music killed the interest in it for me. We did hundreds of weddings.

The most memorable best man/maid of honor speeches were the ones where the speaker showed genuine love for the couple, shared a memory of them/with them, and delivered the speech from the heart, not from a piece of paper. They didn't make it about themselves. They didn't do a "roast" either. Those are tacky.

Honor your friend and his wife (often she's had a vision of this day instilled in her from early childhood) by focusing on them.

If you can't find a way to do humor you could just be genuine and sincere.

In situations like this, you go for very "safe" jokes, trust your gut on this. The day isnt for you, just make some sincere well wishes for the groom and have a good time

The speech aspect of this is really interesting.

My first instinct would be “don’t make this about me.” Think about what you would say about him, how you met, adventures and escapades you’ve gotten into and out of, and how you’ve grown from who you were when you met into the unique and interesting people you are now.

I think that’s a sly way to talk about it without making the whole speech about it.

Honestly I'd say go heartfelt. Talk about how you met, how special your friendship is, how his new partner enriches it, Etc.. that sort of thing. Maybe a funny or special story about when y'all were kids. Something like that. Just remember the speech is supposed to be about them.

You mention a funny story to roast the groom a bit then a relatively lighthearted thing which highlights a good aspect or two.

The best man speech at my wedding involved a childhood story where I dumped a drink on him because we were arguing about something stupid. Then when I went to go pick them up after they'd been drinking when I still had an exam the next day because others flaked on them and uber didnt exist.

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.

The speech is a chance to say everything you want to say, as long as it comes from the heart and celebrates their marriage, you'll do fine.

IMHO it's totally fine to joke about yourself, as long as you aren't generalising there's nothing anyone can say. Transitioning is your story, and you get to tell it how you want.