Resize DPI in batch

It seems I missed to understand your problem. Do you have images with Resoltion greater than 300ppi? In that case you need to record additional step in action to change print dimensions on longer side to 12in what will automatically set smaller side to 8in. I am assuming that all your images are actually larger of what you need because (probably) you set print dimensions to 12 x 8 in what caused resolution to increase or you have missed point in resolution that it matters a lot when working with physical size not pixel dimensions.

If you have all images landscape then simply record additional step with Resample turned on and change Width to 12in. That is it, it will work with assumption mentioned above. If you have mixed orientation then you must record one more action which will be triggered by conditional step if image is vertical orientation. In that case, keep only step to change resolution and add conditional step to run action with single step to change either width or height depend on orientation of your image.

In case that you have all images in the same orientation then you can use easily and Image Processor Pro, just change unit to Inches, type units you need plus keep Resolution to 300.

I missed to change Resample to Automatic which will work just fine.

I use ExifTool for such tasks, it does not process image data, only the metadata so it is very fast:

exiftool -overwrite_original -Photoshop:XResolution='300' -Photoshop:DisplayedUnitsX='inches' -Photoshop:YResolution='300' -Photoshop:DisplayedUnitsY='inches' -IFD0:XResolution='300' -IFD0:YResolution='300' -IFD0:ResolutionUnit='inches' -r 'mac path/to/folder or file'

On OSX you can use shell scripts for basic image manipulation, which should be faster than batching in Photoshop because the files don’t have to open. You might try this AppleScript, it resamples the images in a selected folder to 300ppi.

--Resamples images in the selected folder to 300ppi via sips
--Rob Day 2020

set theres to 300.0
set f to (choose folder with prompt "Select folder with images")
tell application "Finder"
	set i to (every file in f)
	repeat with x in i
		try
			set n to name of x
			set thepath to quoted form of POSIX path of (f & n as string)
			--get the file’s ppi res
			set r to do shell script ("sips -g dpiWidth " & thepath & " ; ")
			set cr to word 2 of paragraph 2 of r as real
			--get the 300ppi scale percentage 
			set s to theres / cr
			--get the current width in pixels
			set pd to do shell script ("sips -g pixelWidth " & thepath & " ; ")
			set nw to (word 2 of paragraph 2 of pd as real) * s
			--resample and resize
			do shell script ("sips --resampleWidth " & nw & " " & thepath & " ; ")
			do shell script ("sips -s dpiHeight " & theres & " -s dpiWidth " & theres & " " & thepath & " ; ")
		end try
	end repeat
end tell