First, I'd recommend reading the MadFientist's post
How To Access Retirement Funds Early where he does the math on taking withdrawals including the 10% penalty, and had some surprising conclusions. Sounds like it's right up your alley.
Second, I'd recommend talking to a retirement specialist - your tax person, or someone else (tax attorney?) you can discuss the details of your personal situation with. I'm not a tax guy, and am still surprised by some of the convoluted rules.
From your post, it's hard to say how old you are, but realize doing a Roth Conversion Ladder requires the converted funds be held in the Roth for 5 years prior to withdrawal to avoid an penalty. If you're too close to 59-1/2 to start a Roth Conversion ladder to use it between 55 and 59-1/2, you'll still be able to withdraw from your rollover IRA between now and when you turn 59-1/2 by taking the 10% penalty. This is where having a full Case Study would be helpful... please strongly consider doing that. At least refer to the
Case Study Spreadsheet for your own use.
Think ahead: estimate your post-FIRE annual budget needs, total annual income after FIRE, and calculate taxes owed. Be aware of the tax bracket you'll fall into once you withdraw enough income to meet those needs. You'd have to pay the 10% penalty + regular income tax rate. If you're 52 or 53, start a "rollover IRA" for your 401K. Then withdraw 2x years of income - converting 1 year's income into a Roth for use after 5 years. Do the same thing for the next couple of years until your 5-year-conversion ladder becomes eligible for withdrawal.
Consider your tax bracket. Since you're already paying 10% on the IRA withdrawal, moving from the 15% to the 25% tax bracket would mean you're paying an effective 35% on any IRA withdrawal dollars above the 15% limit (for Married filing jointly, that's $73,500).
And remember... after 59-1/2, you have 10-1/2 years to convert rollover IRA's into a Roth without having to take 'annual minimum distribution' withdrawals beginning at age 70. This may or may not make sense for you - do the math, AND include your health prospects. Everyone's situation is different. Who knows what the future will bring? But if you expect to live to be 90, then converting to a Roth might make sense as the withdrawals from a Roth are tax free, and require no annual minimum distribution.