Yeh the Tuya Developer Platform can be a bit of a nightmare to navigate.
The old app v3.6.1 method actually still does work as I later found out, but I was lead astray by the characters Tuya uses now for local keys.
Previously it’s always been limited to hex characters but seems now its all characters including special.
I had a couple of devices added recently so like I normally do, I got the prefs xml from v3.6.1 on an old rooted phone I kept for this sort of thing.
The old app said it didn’t support the new devices but still listed them and had the details in the xml. However looking at it I thought the local key was encrypted because of all the special characters which made me try using the API, but after finally getting that to work found that it was the actual key all along.
Oh well, live and learn.