Hi cpX, thanks for your response!
I am not familiar with FFMPEG nor LUTs, but I see that there seem to be LUTs for fuji's film simulations:
https://www.dpreview.com/forums/thread/4463001
However, they seem to do this on the RAW file, which I don´t have available.
Are you suggesting that I reverse-apply the VELVIA LUT and then forward-apply the ProNegHi LUT?
PS I have learned my lesson and in the future, I will definitely save in RAW for important pictures.