You don't need to buy them again! You can take the DLC files from your Steam Our Life folder and put them right into the Mac version of the Itch Our Life game folder. It'll still work to unlock the DLC events.
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Yep!
- The dlc files are in the Program Files > Steam > steamapps > common > Our Life Beginnings And Always > Game
- Send those files to your Mac
- Copy DLC the files, they should .rpa in the title
- On your Mac, after you've downloaded macOS ver of Our Life Beginnings And Always,
- Go on your Finder and open Downloads > right click on Our Life Beginnings And Always > show package contents > resources > autorun > game
- Paste those files into the game file.
If you still need help, dm me on discord, my username is ryeisacatslave :)