My 403(b) doesn't allow Mega Backdoor Roth conversions - should I do Roth 403(b) now?

what you are talking about is not really Mega Backdoor Roth (that would be 401K or 401A in-service conversion to Roth IRA). Just traditional to Roth 403b in-service conversion. You can still do this effectively at 59.5. To decide whether Roth is better than Traditional contributions (very frequently discussed topic) you should first consider your marginal tax rate now vs. average tax rate in retirement. There are other considerations, but you can start there. For most people in the peak earning years "traditional" contributions makes more financial sense - pay lower taxes in retirement than high marginal bracket tax now.

My 403(b) doesn't allow Mega Backdoor Roth conversions - should I do Roth 403(b) now? These two are not causally relevant to each other. You should choose between Trad vs. Roth contribution based on your expected tax rates now vs. in retirement. Read why you. Even for someone whose retirement plan supports after-tax contributions and in-plan conversions (the two legal ingredients required for MBR), the first $24.5k space is still between Trad vs. Roth. After-tax doesn't matter until it is filled up.

Not having access to a MBDR is completely irrelevant to the evaluation of Traditional vs Roth 403b contributions. The Roth vs Traditional thing can be confusing. Review how tax brackets actually work. This video explains the progressive nature of tax brackets. https://www.youtube.com/watch?v=VJhsjUPDulw Then, once you have a handle on the progressive tax system, read this below to help connect the dots on why optimizing tax deferred assets may lead to the most tax efficiency over one's lifetime. It is also why converting tax deferred assets to Roth during one's working years may not be tax efficient. https://reddit.com/r/personalfinance/comments/10qwnrx/why_you_should_almost_never_contribute_to_a_roth

It comes up almost daily, and the answer is almost always Traditional 401/403/457 is better, coupled with a personal backdoor Roth IRA. Now, there are a couple caveats. One, will you have significant income streams in retirement, such as a nice pension. That sets your base salary higher and makes those pretax contributions less desirable. Two, when will you retire? If you save enough for retirement, say $2-3M at 50 years old, but keep working another 15-20 years, then you will have way too much and will face higher than expected taxes in retirement. But if you choose to retire when you have “enough”, then it’s not a problem at all and you will have those 10-15 years to do any necessary Roth conversions.

If you are a supersaver, project how much you may have at 73 and estimate how much you would have to take in RMDs. If the RMD combined with other retirement income is above how much you project you will want to spend in a year and that it would push you into the territory where you have significant IRMAA penalties, then I would strongly consider directing some of the savings into the ROTH to reduce the RMDs.

If you're in a relatively low tax bracket today and expect to earn more over time, I'd lean Roth. The bigger question isn't the lack of Mega Backdoor, it's whether paying taxes now is cheaper than paying them when you start withdrawing decades from now.

What's your marginal and state rate? The upfront deduction may be worth more.