I've dabbled with ND filters - at one point owning a set of square 3, 6, and 9 stoppers. Under most circumstances, I'd use the the 3 stop, very occasionally the 6, and can count on one hand how many times the 9 came out.
I didn't use them enough, so "downgraded" to a variable. I still barely use it, but at least it's a £35 waste as opposed to £120.
Variables can be hit and miss, but both my fixed and variables were from K&F and they've been great. No noticable colour casts or weird "X" that you get in poor variables. No complaints whatsoever.
Same goes for my black mist. It's great, and is glued to my 27mm 1.2.
It's the K&F 1/8 - K&F are apparently a little weaker than others, so it's super subtle. Unless you have very bright specular highlights, or are shooting lights at night, you wouldn't even notice it.
For both the variable and the black mist, I got the K&F "nano k" series. They have a few series - K, X, etc... The only difference is coatings. I think the K series is the lowest spec, but it's been fine for me.