Here's ZF's recommendation, updated January 1, 2022:

5-, 6-, 8- and 9-speed as well as 4HP20 automatic transmissions:
ZF 5-, 6-, 8- and 9-speed as well as the ZF 4HP20 automatic transmissions are filled maintenance-free with specially developed
semi-synthetic ATF oils. However, due to the many factors influencing the service life of transmissions in individual operation, ZF
recommends an oil change after 150,000 km for its transmissions. In operating conditions with high temperatures and loads, or
with unknown vehicle use in the past, it can make sense to change the transmission oil at shorter intervals.


https://aftermarket.zf.com/remotemed...e-ml-11-en.pdf

150k km is roughly 94k miles, so in line with the transmission shop's recommendation earlier in the thread.

When I did my research 2.5 years ago, there was also a six year recommendation. As I noted earlier, my fluid looked pristine at 15.5 years, ~40k miles.