The task can be completed with exiftool (for updating EXIF) and ImageMagick (for crating thumbnails).
First let's download and install/extract both programs. Put exiftool to any folder and include it to PATH (Imagemagick folder was included to PATH during instalation). Next run cmd change to a folder with images and run
for /r %a in (*.jpg) do (Magick convert "%a" -resize 256x256 thmb.jpg & exiftool(-k).exe "-ThumbnailImage<=thmb.jpg" "%a")
or with % doubled to be used in .bat
for /r %%a in (*.jpg) do (Magick convert "%%a" -resize 256x256 thmb.jpg & exiftool(-k).exe "-ThumbnailImage<=thmb.jpg" "%%a")
convert creates 256px thumbnail thmb.jpg and exiftool writes it to original .jpg
exiftool copies original file to .jpg_original which is supposed to be deleted after checking that the update was completed succesfully.