But Hasselblad, Heliopan, B+W, & Leitz filters are available used in great condition for much less.
Agreed.
I've bought used beautiful Hasselblad, B&W, Heliopan, Leica and Zeiss used and Hoya HMC when I can no find what I need in the above mentioned filters.
I also used Kodak Gels and love them for their usability and light weight when carrying a smaller bag.
Used filters are, in my buyer's history, a great savings, though I'd be buying new if I had deep pockets, just 'cause they are easily found on line, when you can no find, excellent used.
I suggest picking three filter sizes and get brass step up or step down rings, in 52mm, 67mm and 90mm to 76mm, if your filter range is covering a vast number of filter/lens hood sizes.
This is possible to achieve over the long term through quality used filters and 4" (100mm) gel filters and smaller.
Currently, the largest filter I have is to the Hasselblad 500 C T* lens I'm still hoping to repair, though I do no see much more reason to think I'll have larger optics to deal with.