@jupiter_rowland ¡Ouch! Okay, so working within those limits, you could:
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Toot the first toot with the image and brief alt text.
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Toot a second toot with just the full image description.
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Edit the first toot to provide a link to the second toot.
The issue about link text is not, to my knowledge, solvable within current Mastodon. I need to file an issue if one has not already been filed. Obviously, you can't do anything which is not supported by the Mastodon code.