Montecfo
Give me a museum and I'll fill it. (Picasso) Give me a forum ...
With respect, at all ages I think you have to look at tax rates, not just earnings.You want both Roth and traditional so you can have more control over your income. Things like ACA insurance, tax torpedo, IRMAA, RMDs benefit from lower taxable income, but you still need some income for ACA and you want enough income to at least use up your standard deduction every year.
I tell my kids when they are young and not earning big bucks, they should contribute to a Roth and in their peak earning years they should contribute to a regular 401k.
These single taxpayers pay higher rates than MFJ.
If you marry and one stays home with kids, THAT could be a favorable bracket. But also a time when funds are tight of course.
But again it is hard to know even that so many years in advance.
But understand that this is only dealing with pre-tax versus Roth. If you max out your pre-tax opportunities, the case for Roth versus taxable is a bit more clearly in favor of the Roth.
That is the better way to address the desire for tax diversity: max out pre-tax then fund Roth to the extent you can.