OK, based on that blog post, I have the following invocation. This doesn't turn the fonts into images but it does downsample all images to 150dpi if they are at 165dpi or larger, and it forces JPEG if there images are uncompressed or PNG (I think).

Let me know if there's anything else you can think of.

function pdf-shrink --description 'Turn images in PDF files to 150dpi JPG'
# https://ghostscript.com/blog/optimizing-pdfs.html
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH \
   -dConvertCMYKImagesToRGB \
   -dGrayImageFilter=/DCTEncode -dColorImageFilter=/DCTEncode \
   -dAutoFilterGrayImages=false -dAutoFilterColorImages=false \
   -dDownsampleColorImages=true -dDownsampleGrayImages=true -dDownsampleMonoImages=true \
   -dColorImageResolution=150 -dGrayImageResolution=150 -dMonoImageResolution=150 \
   -dColorImageDownsampleThreshold=1.1 -dGrayImageDownsampleThreshold=1.1 -dMonoImageDownsampleThreshold=1.1 \
   -dPreserveHalftoneInfo=false -dPreserveOverprintSettings=false \
   -dTransferFunctionInfo=/Apply -dUCRandBGInfo=/Remove \
   -sOutputFile=$argv[2] $argv[1]
end