SlurpingTurtle,
First, congratulations on the progress towards FIRE. IMHO, when looking at paying a mortgage early, there are a few things to consider. First, how diversified is your overall portfolio? If the bulk of it is these properties, consider if you really want to tie up a the remainder of your liquid assets. If you have some sort of emergency, real estate is among the least liquid investments and can only be accessed via a loan or sale. A number of years ago, I met a school teacher who had amassed eight rental properties while working and was living a comfortable retirement with the rental income supplementing his teacher's pension. Then the Northridge earthquake hit. He suddenly had eight un-inhabitable buildings in need of significant repair.
If you've got "extra" money, consider investing in something else (REITs, bonds, low cost etfs, etc.). You can invest for your choice/mix of security, income and/or growth. In many cases you should be able to at least match the interest being paid on the mortgages.