You can withdraw from any account at any time for any reason, there are no restrictions.
I can't tell you about "already taxable" funds in a 401k. Is that because you have taxable money in a pre-tax 401k, or is it because you have a pre-tax account and a designated Roth account, both at the 401k trustee?
When you reach your RMD beginning age, for purposes of determining the minimum withdrawals required, you consider all traditional IRAs combined as one "individual retirement arrangement". You calculate the RMD amount from the combined balance, but the actual withdrawal can occur from only one account, or a mixture, as you choose. Just so long as you withdraw at least the minimum amount from your combined IRAs.
Roth IRAs do not have an RMD and are not included in the RMD calculation for your traditional IRAs.
The pre-tax 401k also has an RMD requirement that is calculated separately from the traditional IRAs and must be satisfied from the 401k.
Roth 401k accounts have an RMD requirement for 2023 and earlier, but the requirement was eliminated for 2024 and forward.
