Image CONVERT=jpg NOLOSSLESS QUALITY=70 HERE REPLACE
That will overwrite the files without prompting. Remove REPLACE if you want to be prompted first.
Remove HERE if you want the original files to be left alone, and the new files to be put in the destination folder instead.
Change the QUALITY value as desired. It's out of 100. You can often set it quite high, e.g. 95, and still reduce file size by a lot from what comes out of a camera.
For how to use the command on a toolbar: How to use buttons and scripts from this forum.
To use the command in a context menu:
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Settings > File Types
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Double-click File Type Groups > Images (or find the JPG type if you only want it on that type of file).
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On the Context Menu tab, add the command there.
(Exact dialog depends on if you're editing the group or the individual type. If there's a Type drop-down in the dialog, choose Run an Opus function. The Function drop-down will always be there and should be left as Standard Function (Opus or external).)
Image command docs if you need something slightly different.