Suggestions for monthly allocating to debt vs investing

Looking for some feedback on our current allocation of money towards debt and investing. We’re fortunate to be able to tackle a good chunk of both simultaneously.

Starting with debt. We started out with about $600,000 of student loans and have worked that down to $435,000. The interest rates are 4.78% and 5.625%. For the last year we’ve been putting $12,000/month into student loans. The minimum mandatory amount per month for the student loans is $6,500.

We also have an expensive mortgage in a HCOL city that is about $12,000/month at 5.875%. The rate is a 7 year ARM and we’re about 1.5 years into it.

For investing, we max out 401k, HSA, and backdoor Roth IRA. Additionally we have automatic recurring investments every 2 weeks on payday with $2,000 going into our taxable account, $300 going into a separate taxable account set aside for our infant son, and $275 into a 529 account. All of these investments are in low cost total market index funds.

So investments outside of retirement accounts are about $5,150/month and additional debt payment is $5,500/month. That split between investing and debt repayment feels reasonable, especially with all of our tax-advantaged accounts.

Right now the amount of money going into the student loans feels too high. We recently refinanced from a 6+% loan to the 4.78%. Originally I wanted them gone as fast as possible. But now that seems less prudent and I think having them paid by the end of the ARM on the mortgage is a good target. That only requires $1,200/month extra to student loans over the minimum payment. And that leaves $4,300 to allocate elsewhere. My gut says to put it towards principal on the mortgage. That extra repayment for the life of the mortgage would save over $1,000,000 in interest (not accounting for changes in interest rates). That plus having more equity to refinance in the future seems like a better return than adding more to the stock market.

How would you allocate in this situation? Really appreciate any feedback on our current plan.