One thing you should get out of your head is that there is zero risk. Even keeping it all in cash in a checking account has risk.

CD ladder isn't a bad choice. A treasury bill ladder could also work and might actually fit as well. The risk is in a rising interest rate environment the t-bills you buy now will be worth less in the future. You'll still get the dollars in absolute terms but it might buy less. Too bad you can't buy I-bonds past 10k a year.

Also the biggest risk in your plan is the "leave alone and not manage". There is manage a little, manage a lot, or pay someone to manage it for me. Leaving it alone and not managing it has its own risks.