hi there,
I recently bought a 6a and did the ota to be able to unlock the bootloader and went on installing grapheneOS right away ignoring the availabe upgrade to android 13.
should I revert to stock to download all ota and re-install grapheneOS or are there no disadvantages anyway?
thanks in advance for your help!

No. From what I understand, the advice to make sure your phone is up to date before transitioning to GrapheneOS is in part because you might run into issues during installation. One big issue and its fix is covered in this post: https://discuss.grapheneos.org/d/644-web-installer-improvements-safer-flashing-method.

Since your new installation of GrapheneOS is working, there's no need to go back to Stock.