Your phrasing of "401k to Roth IRA conversion ladder" is compressing several steps. Do you know if your 401k administrator allows partial withdrawals? They may not. Also, is your 401k Roth now, or traditional? Are there any after-tax contributions in the traditional?
What I would recommend is simply to roll over your 401k to and IRA with Schwab. This should be very routine for both entities. (And initiate it through Schwab; they are more motivated to take on your account than your 401k administrator is to drop it) If you have any after-tax contributions, you can have them rolled over into a Roth immediately, without taxes. And you can only do that at the time of rollover. Then, you can convert to Roth at your leisure.
This could have consequences if you are also doing a back door Roth IRA contribution. All your traditional IRA balances count when considering taxable gains, even for IRA (but not 401k) accounts that are not the source of funds. But, you only mentioned a Roth IRA, so no trouble with that.