Some disjointed comments...
One, don't let IRMAA loom too large in your imagination. There is only one of you, so the dollars involved are only half what they are for a married couple on Medicare. And the first tier doesn't really cost that much, $1148.40 total in 2026 and an estimated $1234 in 2027 assuming 2.25% inflation. Even the second tier is only $2884.80 in 2026 (totaled for all 12 months for Parts B and D).
Second, shifting stocks from tIRA to Roth is not "paying a high price." While I'm pretty sure you can't literally transfer holdings between the accounts, you CAN buy and sell on the same day. If what you hold is traditional mutual funds then you will buy and sell at the same price. If what you hold is ETFs or individual stocks, you could use limit orders set at approximately the current price, if you are worried about your orders executing unfavorably.
You seem to have a lot of cash but if you need to liquidate bonds to have enough in your settlement funds for the stock transactions, since bonds are less volatile than stocks I would recommend selling what you need to in each account (and letting the trades settle) before doing the stock transactions.
I think the benefit of long-term tax-free growth in a Roth outweighs its value in managing IRMAA. If there is no market crash, then you can reap both benefits if your growth investments are in the Roth. If there is a crash, then you might want to give up the IRMAA management benefit (maybe even increase your taxable income to just under the next IRMAA tier via tIRA distributions/Roth conversions or recognition of capital gains in taxable) to allow the Roth time to recover.
Sorry for the long post.