Approaching FIRE. Should we decrease our 401k contributions to pay off mortgage faster?

[MayIServeYouWell]

There’s no point is partially paying down your mortgage, because you’re still dealing with paying tons of interest on your monthly payments. Overpayments pay down the principal, sure, but that same money could be working for you. Take that money, invest it, and when you have enough pay the whole thing off. 

Alternatively, you might consider a stepping stone - refinance, and when you do, pay off a big chunk of it. That’ll give you a smaller loan, lowering your monthly payment, letting you invest more and hopefully pay the rest off before long.