This should not be considered tax or legal advice; but rather, general information/opinion. As others have suggested, consult a qualified CPA. Having said that my views are:
If a 401k or other qualified plan has both pretax and after tax contributions, the prorate rule comes open to play to track and calculate taxes on distributions.
Backdoor Roth doesn’t avoid taxes, it is a way to bypass restrictions on ability to do a Roth due to having too much income.
When you convert from tIRA to Roth you pay taxes on the distribution taken from the tIRA (using prorata if the distribution contains both pre-tax and after-tax dollars)
If you made a non-deductible contribution to a qualified plan (e.g, tIRA, Simple IRA, SEP) in the current year or any previous year you file Form 8606 to track non-deductible and calculate the prorata percent for tax purposes.
Again these are just opinions and my understanding. Please see below.
Single Distribution Rule for Retirement Plans
www.irs.gov