Have you already educated yourself about investing? Do you feel solid in your understanding of the market being volatile, but just investing in index funds and letting it ride for many years? Because if you aren’t there, then first I’d focus on building your education. If you don’t you could handle “losing” half your money temporarily, then you’re probably not ready and/or not ready to invest just in equities.
If you feel you’re ready to invest, it sounds like you could start your taxable brokerage account now, then you can tackle moving your rollover Ira into your job account and also investing in that 401k until you get the backdoor Roth IRA figured out.