Back door roth conversion and dollar cost averaging. Should I contribute to traditional IRA directly, or contribute to taxable account and convert to traditional IRA?

I want to minimize my tax liability. The contribution to the traditional IRA would be post tax monty (I do not qualify for the deduction). My plan is to do the rollover at the end of the year. I believe that the gains from traditional IRA is taxed as ordinary income during the conversion to roth IRA.