A) Converting my non-Roth-IRA to a Roth Backdoor IRA (transferred 401k balances after leaving company) and using this account to pump into mutual funds and ETFs

Or

B) Starting a taxable private brokerage account to pump into mutual funds and ETFs

My issue with converting my current IRA to backdoor IRA is I don't know my exact pre-tax vs post-tax contributions from the job I had where I carried over my 401K into this bucket. What are the right steps to figure this out? Contacting the company?

If I go with option B with using the taxable brokerage account, my worry is annual capital gains payouts in mutual funds or ETFs that occur before reinvestment, triggering tax payments even though the gains aren't really "realized". How do you get around this?

Seems like an excellent time to buy. If I go private brokerage, I don't plan to pull this money out until retirement either unless it's for a down payment on a home or we're in an emergency situation.

Thanks for sharing tips

Just do a search for tax managed accounts for companies like vanguard or fidelity. You will still have some capital gains each year but they do things to try to keep your taxes in a manageable level.

Both.