Premix and Postmix Transactions are not wasteful at all. I recommend reading more about Whirlpool architecture: https://docs.samourai.io/whirlpool/basic-concepts
I just read the doc, it didn't explain at all why you would do a premix trasaction. You can go directly from the deposit account to the postmix account, premixing first just wastes block space (and hurts your privacy).