I'm not sure you can do what you want a do. I think you need to seek the advice of a CPA, enrolled agent, or tax attorney who does complicated retirement planning (not just filing ordinary 1040s).
The normal ("pro-rata") rule is that you cannot take out just after-tax contributions from non-Roth plans. Typically each dollar that comes out will be partially after-tax and partially pre-tax. (based on the 12/31 value of all similar plans: total after-tax contributions ("basis") / total FMV. Usually a very small %.) That is not what most people expect.
See for example the "How do rollovers from other retirement accounts (e.g., 401(k)s from previous employers) into an IRA affect the Pro-Rata Rule calculations and the strategy for Backdoor or Mega Backdoor Roth conversions?" from this article from a company with CPAs offering to help in this type of situation. I have no affiliation with them and cannot vouch for them. I just found this article with a quick search.
https://rgwealth.com/insights/how-to-avoid-the-pro-rata-rule-with-backdoor-and-mega-roth-conversions...
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