first, since your Roth has been open and funded since 2018, then it meets the '5 year test" (calendar years 2018, 2019, 2020, 2021, 2022). And since you are older 59.5 years old, no matter what you withdraw from the Roth, it is going to be tax-free and penalty free.
here is a good website that explains IRMAA. I find that most people I run into in their early - mid 60's have never heard of IRMAA.
https://thefinancebuff.com/medicare-irmaa-income-brackets.html
It might be worth it to employ a financial planner for a few hours work. It could save you an enourmous amount of taxes.
You wrote that you need to exit from the current administrator of your retirement funds. Why can't you simply roll it all over to a Traditional IRA (I am assuming it is all 401k money) and figure out the conversions to a Roth with the planner? what is the rush to do the conversion?
I am not familiar with a 479b...did you mean a 457b?
<<Does the IRA to Roth IRA conversion need to take place within 60 days of my seperation from my employer?>>
NO. there is no need to EVER convert from a TRAD IRA to a ROTH IRA, if that is your desire. You could decide to covert to a Roth many years into the future; there is no time limitation (but once you are subject to RMDs, you must satisfy the RMD requirement before converting dollars to a Roth, but first things first).
