“Question:
Is tanha the cause or condition? Or both depending on the situation?”
Tanha can be cause or condition. Same for avijja. Both are root causes (related to lobha and moha).
Both of them will remain as anusaya until the Arahant stage is reached.
But they are not there all the time. They may be activated by a strong sense input.
Of course, BOTH will be lost at the same time. There can be no tanha if avijja is removed and vice versa.