If you regularly use ND grad filters then I think 4x6" ND grads are the way to go with LF. Re your problem child 90mm lens with CF: there is a press-on filter holder that can accommodate this, but it is expensive and is not a good solution for combining an ND grad with a circular polarizer:

https://www.robertwhite.co.uk/lee-fi...er-holder.html

With this setup I would position a square linear polarizer in front of the ND grad, which would not always provide optimal polarization since it could only be positioned in 90 degree increments, but often that would be adequate. I used this setup mostly with my 8x10 camera and did not experience any significant vignetting issues. I doubt you would see much vignetting with your 90mm lens with CF on 4x5 with just an ND grad, but suspect you might see some when you add the square polarizer on top.

Frankly it would be more practical to either accept the light falloff without the CF or purchase a smaller lens.