We have several websites that have the same tracking structure and always use the same tags, triggers and variables. Is there a way to create a code snippet centrally so that it can be called for all containers? Or what option would be conceivable at this point?
Apart from using the same container code for all of the websites - not really.
You can also use a meta setup (and keep the separate containers) but the Piwik PRO tracking tag can’t be added there. In theory I think it should be possible to use this method but it’s not very user friendly (using code instead of UI). Templates such as goals, custom events and ecommerce events work fine with the meta setup - it’s just the main tracking code that has to be in a non-global manner.