I was looking at CD ladders recently through Fidelity. A 2-year ladder averages 4.6% per year. That would take care of your first couple of years. You could still use treasuries or some other option to hold your money for the rest of the time before your pension kicks in.
Definitely DIY the ladders. Fidelity will auto-select ones that can be called, which may not be good in this rate environment. (*someone who buys a lot of CDs via Fidelity.)
As to op, I would sit down with a CPA. I've helped manage retirements, and the top two issues are: (1) managing taxes (thus the CPA) and (2) large, unexpected/unplanned expenses (medical, car, injury, soaring insurance costs, etc.). Plan out various scenarios, including some worst-case scenarios and see how they look. You also need to consider how much in savings/EF you need rolling into your traditional retirement as well and plan now for that.
The treasurys route would be tempting if I were setting this up for someone today: you keep the corpus of the savings and rates aren't bad now. You may also want to throw in some i-bonds in there for the early years especially, with rates where they are - no downside there (after the first year, where those aren't liquid). I manage one retirement with a large position of CDs and treasurys for that reason.
You also want to consider taxes because you may want to start slowly withdrawing your 401(k) funds, too, in lieu of using the savings, in order to diminish your RMDs and to give you more control over when you have to take income (and thus potentially increase tax brackets and/or tax other income). E.g., you may have to pay more taxes if your 401(k) (which I suspect is more likely in stocks) soars while you're not withdrawing any funds such that you have to take larger RMDs/more income later on in your retirement.
Your numbers seemed a little tight to me given your plan, however, you may want to do a full case study so that we can see the overall picture rather than a few pieces, as time permits.
In short, there's no easy answer or one-size-fits-all here, especially with your unique facts - you need professional advice on it, especially re: the taxes.