Next step after paying off mortgage

That's the crux of the trade-off. Do you want to save on taxes (and up with a bit less flexibility) or do you want to increase your flexibility (and perhaps pay a bit more tax).

One good example of where the added flexibility might come in handy is if you decide to move. I'm assuming from your username that you are in Omaha. So, if you had $500k in the brokerage, you would have enough to be able to liquidate a chunk of that (taking tax consequences into account, of course) and buy the new house in cash. You could move at your leisure and then sell the old house without being a in a rush. The normal way that people handle buying/selling when moving is to make the purchase contingent on the sale. However, if you come in without a sale contingency and with cash, that makes you a much more attractive buyer. So, if your market is competitive that might very well give you the edge.

Of course, there are other ways in which a healthy brokerage balance gives you additional options that the same money in a retirement doesn't give you. But if you don't need/want all the flexibility, then socking money away in the HSA and 403(b) is likely to be winning play for a dual-income couple with no children.

This is extremely helpful! Thank you. My only thought with not putting the bulk of it in the taxable account is that we might need to bring down our income to help with taxes now that we don't have any write offs. Since we don't have any children. That was what was pulling me towards the HSA or 403b. Of course we could split it.

OK. That's tremendously helpful. Since your plan is to go PT in 3-ish years but not start drawing from investments for 10-15 years, I think that there are 2 basic ways to proceed. For me, I found that having 80-90% of my savings in retirement accounts and only 10-20% in the brokerage put me in a situation where I lacked some of the flexibility I would have liked. So, I think beefing up the taxable brokerage would be a good choice. The downside is that if you are a high income earner, then you are in a high marginal tax bracket, and if you aren't maxing the 403(b) and/or HSA but putting money in the brokerage, you are forcing the tax on the money today.

The other possibility is to max the 403(b) and/or HSA for as long as you can and only put money in the brokerage after you have maxed the 403(b), the HSA, and the Roths. This has the advantage of saving on the high marginal tax bracket, but the trade-off is that you have a bit less flexibility. If you don't need the flexibility (i.e, you're not planning to uproot and move across the country or overseas, you're not planning on some expensive luxury travel the world for a year type of thing), then this is probably the more tax efficient approach.

Note that given your timeline, I don't think that you need to jump through hoops for Roth conversions. One of you will be 59.5 in 6-ish years, at which point you just make normal withdrawals from the retirement account as needed.

I'm definitely not a financial professional, so please take the above for what it is: the rantings of a random guy on the Internet.