Hello @3Adigit ,
I will try to be short and precise.
1. Do I have to manually assign products to the profiles?
A: You can use our app Auto Shipping Profile: Shipr to do exactly what you want. Using our app you can create rules based on product attributes to bulk assign products to shipping profiles.
- How can I make the rates not add up but show highest rate only when items from multiple shipping profiles are present in cart?
A: Sadly this is not possible in Shopify. Shopify can only add rates.