My 401k HAS to be moved to a different firm or arrangement

No, it’s not.

Right there in the OP:

Rolling one 401k into another is quite common and allows future backdoor Roths without any additional taxes.

That’s fair - not sure why I let that slip my mind. Either way, if the OP desires to convert a large portion to Roth via a back door, having control of those funds in an IRA that allows for greater control, larger selection of investments and (likely) lower fees, an IRA is likely a far better place for those funds. Assuming they aren’t running into contribution limits in their 401k.