Like I told the other user, I actually very frequently don't do true alt text, but instead write a transcription in the body of the post. I do it using the Lemmy spoiler syntax:
Transcription
[the transcription here]
But one could also just write the transcription directly, especially if it's relatively short and unlikely to obstruct sighted users' experience too much.
It's unfortunate that some apps (including the official one, Jerboa) don't support the official "alt text" field, and they definitely should, but even the web's alt text field is rather limited to use for full transcriptions.