Hi u/MuppetParty - I recommend the brand new $1,499 Canon EOS R7 from Amazon or Adorama, a $499 Tascam CA-XLR2d-C XLR Microphone Adapter - plus a $399 EF to RF adapter with a drop-in variable ND for your Canon glass.
This $2,397 setup will cost you a little less than picking up another 6K Pro body -and a lot less than a $3,898 FX3.
With this rig, you will have phantom powered XLR inputs, a variable ND, IBIS, lightning fast autofocus, a fully articulated LCD, a headphone jack, CLog3 plus 10-bit 4:2:2 recording - and you wouldn't have to buy new lenses.
Here it is, shooting with EF glass:
Canon EOS R7 cinematic video, canon log 3 graded, Tokyo early morning by Ramen Papa's Chill Video (4K)
Canon EOS R7 Camera Test // Canon EOS R7 Video Test 4k WEDDING B ROLE by SIDE HUSTLE VIDEO VLOGS (4K - shot with EF 50mm STM, EF 24-70 mk2 and RF-S 18-150mm IS STM)
A great value for your money.
The only downside is that the camera and the XLR adapter are both back ordered, so you may have to wait a few weeks to get them.
Hope this is helpful and good luck finding the right B cam for your 6K Pro!