About the only thing which would be to remove any excess folders from /config/plugins that are from plugins which no longer installed. Realistically though, most plugins would automatically remove their directories (or should). However, there are also "system" folders (eg: dockerMan) which cannot be removed
To get rid of excess templates for docker containers, go to Apps, Previous Apps and delete (check off and then delete) the ones which you are no longer / have no need for anymore