First, look into a backdoor Roth. Are your old 401(k)s still with your prior companies, or are they in rollover IRAs? If the latter, see of your current employers allow you to roll them into your current plans. If you don't have any other outstanding tIRAs, it's very easy to open a tIRA and roll it over immediately into a Roth. And even if your wife is not employed, I *think* you can do a spousal IRA for her as well (doublecheck, though). If you can swing another $11K/yr in tax-sheltered accounts, that's a help.
Second, personally, I would recommend changing the order of things: invest first, then pay the mortgage down if you decide to when you are closer to FIRE. The thing about the power of compounding is that it gets more and more powerful over time -- a 7% return will give you 2x your investment in 10 years, but 4x in 20, and 8x in 30, whereas a 4% return will give you not quite 1.5X in 10, 2.2x in 20, and only 3.25x in 30. So the math says it is more beneficial to hit the highest-return option earliest, so that it has a longer time to ride.
Stupidly simple example with no bearing on reality, for illustration only: Assume 20-yr FIRE deadline. Say you have $100K now to do something with, you owe $100k on a 4% mortgage (no minimum payments, you can throw a chunk of $ at it whenever), and in 10 years you will have another $100K free to invest or do whatever with.
Option 1: you use that $100K to pay down your mortgage immediately. In 10 years, you use the next $100K to invest and earn 7% on it. That gets you about $200K. So in 20 years, you have $200K and a paid-off house.
Option 2: you invest that $100K. You are not paying the mortgage, so in 10 years, you now owe about $150K on it. But your investments earned 7%, so now you have $200K. Now you take that second $100K, pull $50K from your earnings, and pay off the mortgage. Now you have $150K that sits there for the next 10 years and at 7% grows to $300K. So in 20 years you have $300K and a paid-off house -- that is 50% more invested assets, simply from changing the order in which you invested the exact same amount of money. (of course, the best return comes from keeping everything invested and only paying off the mortgage at the end of the 20 years -- that nets you over $350K).
I recognize that this isn't all about the math -- there is no guarantee the market will average 7%, or even beat your 4.25%. But investing the cash isn't as risky as you think it is (the longer your timeline, the more likely you are to come out ahead investing, because the market almost never loses money over 20-year stints), and prepaying your mortgage isn't as "safe" as you think it is (until you get that last dollar paid off, all the prepayments in the world won't keep you from needing to make that next month's payment).
But here's the kicker to all of this: that 50% difference in investment returns over 20 years isn't just happier numbers on your account statements -- it is a direct measurement of the life-hours you need to spend before you can FIRE. Replace the $200K/$300K with $1M/$1.5M. $1M and a paid-off house will allow you to FIRE on about $40K/yr, which most folks here could manage just fine. But what if your FIRE budget requires $60K/yr? Investing now and paying the mortgage later -- what you are seeing as the "risky" path -- gives you a free extra $500K, simply through the value of compounding. On the flip side, being "safe" now means many more hours of work to earn enough to save the extra $500K you gave up in your flight to safety. Is your need for perceived security worth that number of additional life-hours you will need to spend away from that wife and that lovely child?
Tl;dr: you are 28 years old and have awesome earning power and really solid savings. Better yet, it sounds like you work in sales, which means you will always be able to find a job. This is the time in your life when you can afford to be aggressive -- and when doing so is likely to bring the biggest long-term benefits.
