Since you are already maxing your 401k and HSA and have a super solid EF (great job btw!!) I would probably split the “extra” between the SLs and investing in a taxable account. 3.89% guaranteed return ain’t shabby (I wouldn’t pay the mortgage early unless you have PMI in which case get rid of that ASAP and then resume making standard payments) but the market will outperform it over the long haul.
Of course, it’s a very personal decision too, some people value being free of debt as fast as they can, and for them it makes more emotional sense to pay off any loan (especially non mortgage loans) before saving taxable money above an EF. Others prefer the flexibility of cash and tangible assets over less debt (if crap hits the fan you can’t get early SL payments back to your account, you can sell stocks though potentially at a loss), or simply choose to invest as much as they can while it has the maximum amount of time to grow because the math makes sense. I’m a middle of the road person myself so would probably split the contributions at something like $1000 SL/$1400 investment, and like many here recommend Vanguard for the latter.
To be clear, no option is wrong- be it 100% SL, 100% invest, or a split of your choosing- it’s just making the choice that you are most comfortable with.