i did a small speech last year at our wedding, and did it in thai. i got a few laughs in the right places, so some of it must have been understood by some π
i wrote mine down in phonetic, and read it from my paper on stage
i am not sure you need to learn how to read thai quite yet. just write what you want to say in english, get it translated, write it in phonetic, practice it to someone and see if they can follow what you're saying.... repeat, repeat
good luck. i was really pleased i did mine, but as always, a good wedding speech is always quite short