So, I would suggest a slight tinkering -- I am going to focus on 2019, for 2018 you should see what you can manage:
1. Max out your tIRAs. Set up an automatic transfer into your IRAs as of January 1 so you don't have to think about it. VTSMX or the equivalent -- set it and forget it.
2. Max out your 401(k)s. Both of you. You have sufficient cash on-hand to cover you through an emergency. Trust that safety net and your ability to hustle.
3. Throw everything else at the 6% SLs, and refi if you can to a lower interest rate.
4. Once the SLs are paid off, begin throwing that money into a taxable account (also in VTSAX or the like). As discussed above, the Roth pipeline requires you to have @5 years expenses saved and accessible. If you can get the loans paid off in a few years and then build your regular post-tax account, this will give you the cash you need to do that.
5. Drive the car into the ground. If your wife wants to keep it a few years, that is the same period in which you will eat the most depreciation. You have chosen a car that should last 15-20 years, so your new goal is to keep that puppy healthy and running as long as possible to get the best value out of the expense.
I cannot express strongly enough how important #1 and 2 are, especially at your age. Yes, your loan rates are likely comparable to what the market will do over the next few years, or perhaps even better. But as I posted earlier, you pay those loans with post-tax income, so you will have to make $130-150 or more just to pay off $100 of loans. I would suggest doing a rough cut at your taxes both ways: what do you pay in federal
and state taxes if you don't invest in the 401(k)? (Remember, the great state of NY also gets a @6.5% cut). And how much more money do you have in your pocket if you do max those out? That extra money is all cash that you can throw at the student loans! You literally get more out of each dollar you make.
Look again at
@MoseyingAlong's post: maxing everything out would cut your federal taxes to around $2500! Skip the 401(k)s, though, and your taxable income pops up to $61K, which a quick web calculator says means $7K in federal taxes (plus whatever additional you'd pay in NY State tax -- what's that, around $2K difference?). So you can invest $37K and still have
@$6500 to throw at your student loans!*
For your DW's fears, I suggest looking at the real downside risks. The reality is that throwing all your cash at the student loans is actually
more risky until the loans are completely paid off and you have built up a taxable account. Once you decide to write a check to the loan company, you can never get the money back if you need it later -- it's gone. OTOH, with a 401(k), you can frequently take a loan from the account, or even withdraw the money and just pay the taxes/penalty. So if one of you needs a $50K surgery, your 401(k) can provide that cash, but your paid-down student loan cannot. In addition, most employers let you change your 401(k) contribution very frequently -- so if the shit hits the fan, you just stop your automatic payments and at least have access to that extra cash each month again, which together with your EF may be enough to get you buy.
Anyway, let me join in the congrats on the huge progress you've made so far -- really great job in a very short period of time. Just don't let your fears lead you to making decisions that lose you money and are actually more risk in the short-term.
*Yes, the loans are deductible -- you get up to $2500/yr in interest deductions. But, (a) that is an incentive
not to pay them off, because you lose the deductions as your interest paid drops below that threshold, and (b) it is largely meaningless, because you're going to take the $24K standard deduction anyway.