If you're stuck with tricking them to fire at full power for consistency, you can get ND gels to reduce the flash power - make sure there's sufficient airflow between the flash head and the gel as full power may end up pretty hot. Regular ND filters on the camera would also work, or you could stick them between the flash and the slide (they'll be large enough to cover the slide up close)
You can also diffuse with copy paper or parchment paper or just about anything whitish, a few layers can really cut the power down and provide more even illumination, like a light table. Colour shifts can be an issue but if you're setting up something semi-permanent for slide copying you only have to sort it out once.
Finally, moving the flash back will also help, assuming you have space or are allowed to knock out a wall or two
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But yeah, auto-thyristor and copy work requiring consistent results can be a little piece of madness...