How to customize / edit the filter presets in Inkscape?
Are the filters presets unchangeable or are there ways to customize their values?
For example, maybe change their strength or the direction of the bevel.
How can I alter the filter presets in Inkscape?
You can edit those filter presets once applied on an object using Filters- Filter editor(refer image). Vary the settings to get desired result.
Change the effect parameters & you can view the live changes in an object.