@james If money is really not an issue, here are two expensive but good options:
Most non-native colleagues who speak German really well seem to have taken in-person Goethe-Institut classes in their home country. They are available in nearly all countries except like North Korea or something.
Some of my colleagues have taken Berlitz classes in Germany. I see online classes on their website, including 1 on 1, that would be very effective: https://www.berlitz.com/en-de/languages/german