Not entirely sure what docker version is on Synology, but this link shows some problems with upgrades to 16.x: Updating from 15.11 to latest docker build fails - #9 by paula.kokic
Did the upgrade finish successfully? What was the output from the upgrade logs? First we need to make sure the upgrade actually finished successfully before concentrating on Redis. Either way, unless docker is of the version in the link above, then you won’t be able to go to Gitlab 16 on Synology until Synology upgrade the docker version to a supported one.
You’ll need to check that though, as I don’t use docker on Synology.