So what would the math look like trying to utilize the saver's credit? I am unmarried. Michigan's income tax rate is 4.25%.
Below is information I found from the IRS' website.
2019 Saver's Credit
Credit Rate Married Filing Jointly Head of Household All Other Filers*
50% of your contribution AGI not more than $38,500 AGI not more than $28,875 AGI not more than $19,250
20% of your contribution $38,501 - $41,500 $28,876 - $31,125 $19,251 - $20,750
10% of your contribution $41,501 - $64,000 $31,126 - $48,000 $20,751 - $32,000
0% of your contribution more than $64,000 more than $48,000 more than $32,000
And just in case the Roth 457B happens - probably wouldn't do that personally, and just be sure to know how the specifics of how Roth 457B's work before making your decision. Sounds like a great idea for an early retiree at first glance, but when you realize the "457-ness" only applies to the penalty for early withdrawal and you still have to wait until 59.5 for tax-free withdrawals and it is like a 401K in that if you withdraw directly from it you're hit by the pro-rata rule, it isn't quite as great as it might seem initially.
The way to use it would be either as additional Roth space for 59.5+ or to plan on rolling it over to a Roth IRA where you get more favorable early-withdrawal rules. I'm personally in the very-pro-traditional camp though.