You could add a custom yes/no column (#metaedited/MetaEdited), select the books up to 7348 and Set MetaEdited to Yes. Define a virtual library (2B Metaedited) with search term not #metaedited:true.
Then, as you edit the metadata you would set the MetaEdited column to Yes.
There are too many coincidental ways in which date modified gets updated, eg if you correct a misspelt Tag all books with that Tag get their date modified reset. I have a Last Changed custom column that I set, usually by entering '=' which sets it to 'Now'. If I correct a misspelt Tag my Last Changed date stays as is.
BR
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