Let the message be edited after merge operation.
After the merge operation, allow the user to edit each draft email, one by one, so that small personal messages could be added. This will make each message unique and personalised.
Debby Kurti commented
You can accomplish this by adding a "comments" field to your spreadsheet and then inserting <<comments>> in the appropriate place. I do this when sending out grade reports to my students and it works well.
Alexandre Plennevaux commented
You could easily do that just by having an additional column "personalmessage" and then write the personal message inside each column cell. Then in the template <<personalmessage>>