a >$100, 15 minute DIY job. Which brand German car? It's probably time for a trans fluid swap, which is essentially what dropping the pan and replacing the filter will also accomplish, but you can get 95% of the benefit by just swapping the fluid. Skip the decarbonization unless you plan to do it every 30k miles. It just builds up again, but only to a point. For anything else, like the secondary air valves, just replace them when they go bad. You'll know when the secondary air system is out of whack when it is either really difficult to start from cold or it runs really poorly for the first 15s or so.