I don’t really understand why this is a concern with docker. Are there any particular features you want from version 29 that version 26 doesn’t offer?

The entire point of docker is that it doesn’t really matter what version of docker you have, the containers can still run.

Debian’s version of docker receives security updates in a timely manner, which should be enough.

You are adding a new repo, but you should know that the debian repos already contain docker (via docker.io) and docker-compose.