I'm surprised no one has mentioned mega backdoor Roth (which is different from just a backdoor Roth). IMO, if you want to diversify your pre- and post-tax savings, this is the way to do it.
This allows you to save up to an additional $43,500 after tax, on top of the $22,500 ($30k, if you're 50 or older).
The catch is, your employer's retirement plan has to offer/allow in-plan distributions. I started doing this just this year.
I max out my traditional pre-tax 401k to get the tax break. Then I make after-tax contributions to that same 401k that get a rolled over to a Roth. I'm nowhere near the $43,500 that is allowed (more like $8k for the year), but I can increase it at any time.
My 401k is with Fidelity, and I just called them and told them I wanted my after-tax contributions to go to a mega backdoor Roth. They knew what to do and set it up all behind the scenes. They will also keep track of the paperwork so that taxes are correct when I finally start taking distributions. The only "catch" is the contributions go to the same fund as my traditional 401k. I can't direct them to other investments.
I had a "regular" backdoor Roth (with the $6500 contribution) for many years. But I rolled over an old 403b to an IRA and can't do that anymore, so I do the mega backdoor instead (which I think is better anyway). Link below that gives a good explanation of it all.
https://www.nerdwallet.com/article/investing/mega-backdoor-roths-work