Unlike Germany Tanzania doesn't need the labor so no need for guest worker
Well, we may be surprised. We can look at the nearly 200,000 Indians in Kenya which even has less natural resources than Tanzania. IOTL we saw Indians dominate the trade sector in Zanzibar, and so just as Indian migration to Kenya and Uganda was sparked by the Uganda Railway, I don't think it's out of bounds to expect a diamond mine to open in Tanzania in let's say 1925, and then some Indian Zanzibar traders to arrive as 'middle men,' and that being the trigger for a flow of hundreds of thousands of Indians into mainland Tanzania in the next 40 years. As for labour, Indians in Kenya usually worked as traders and opened businesses, and proved remarkably resourceful in terms of employment, so I'm sure they'll be fine.