Mega backdoor Roth or Taxable account

[vas]

I'm making the maximum contributions to my 401k, 401k catch-up, HSA, and backdoor Roth accounts. I can (but haven't so far) contribute up to 20% of my salary to a Roth IRA using the mega backdoor Roth technique. I probably will not save that much until the 529 is filled with an undergrad STEM degree, so I have more taxed advantage space than I am able to use. In this situation, is there any reason to utilize a taxable account for some savings? This is all retirement money so I'm thinking about tax diversity etc. Given the flexibility of the Roth I can see no reason to do so. Am I missing anything?

[Bill M]

I once had a financial planner tell me, "Don't do anything with everything." I think it was very very good advice.