What to do after maxing tax advantaged space

Me (30M) and wife (29F) are maxing all of our tax advantaged space and are trying to figure out how we should be allocating the remainder of our savings. We aim to retire early in 10-15 years and would like to set ourselves up for success in the best way possible.

I made a very similar last year, but many of the answers were along the lines of "your mortgage is less than tbills, so don't prepare the mortgage". The math here has obviously changed with how rates have evolved, so I'm curious others thoughts in the current environment

Income: 775k this year, this is set to drop to about 625k next year (the nature of equity-heavy compensation unfortunately). We both work in tech, so the future of this income is far less certain than for docs given the state of the tech industry. We've only been making this high of an income for a couple of years.

Assets (~1.8M total):

• 200k home equity (575k remaining on mortgage at 5.375%)
• 1M in retirement accounts (401k, roth IRA).
• 400k in brokerage account
• 75k in 529
• 50k in HSAs
• 100k in cash

Automated Savings:

• 138k in 401ks (we both have access to mega backdoor)
• 14k in Roth IRAs
• 8k in HSAs

We will save another ~120k this year aren't entirely sure how to allocate this. We see four primary options:

• 529s. We plan to have 2 kids in 3-4 years and figure that the longer the money stays in these accounts the more we benefit from tax free compounding. This is obviously weighed against the risk of overfunding the account (and hard to say what higher education will look like or cost in 20+ years). Our state gives a tax deduction for the first 20k of contributions and our state taxes are around 5%. We are committed to fully funding our children's undergraduate (and possibly some graduate education) as this is what was done for both of us.
• Prepay our mortgage. A 5.375% risk free return seems fairly compelling, but some of this return is counteracted by the fact that we itemize our taxes (and if the standard deduction increase is not renewed next year, this becomes even more powerful). This is likely not our forever home, and will likely move into more space in somewhere between 4-6 years depending on our exact timeline for kids.
• Invest in a taxable brokerage account.
• Invest in real estate. This currently does not appeal to us, beyond having a small allocation to REITs as part of our brokerage.

Our current thinking is to do just enough (20k) in the 529s to maximize the state deduction, put another 20k or so into prepaying the mortgage (the idea being this would be a safe return in lieu of having bonds in our portfolio), and putting the rest of the money into the taxable brokerage account. While putting more in the 529s seems more optimal (to maximize tax free compounding time), we have some concerns that we would have relatively little of our NW in liquid non-retirement assets if we went this route given how heavily we are investing in our 401ks with two mega backdoors.

Would appreciate any thoughts or ideas on how best to think about allocating this remaining savings given our situation and goals.