Am I understanding correctly?
-You plan to retire with very nearly exactly 5x in taxable accounts
-You have no (or insignificant) existing Roth contributions or conversions
One missing factor would be tax rates. For example, someone who plans to retire very early may be in such a high tax bracket now that even with the 10% penalty it would make sense to pull funds from traditional or Roth accounts to fund a low expenses retirement.
Another missing factor is age. You talk about a 5 year Roth conversion ladder, so it seems likely that you are more than 5 years from 59.5, but it never hurts to be explicit. Especially since 60/40 in taxable seems pretty conservative for someone who is much younger than 55.
How set are you on retiring in two years? If you stay 60/40 now and haven't met your 5x in taxable goal, will you keep working?
If you are going to panic if this goes down, you should indeed be very conservative: CD's, MM, MYGAs, bond fund.