My impression is, that the crf option is actually (and very unfortunately!) not supported by kdenlive yet. Why? Well I used a rendering profile with crf=20 and rendered. Then I changed the profile to crf=18 (leaving everything else unchanged). The filesize of the first rendering should usually be smaller than the rendering of the second file. BUT: It is not! The two files had the exact same filesize! Therefore it seems to me, that kdenlive actually just ignores the crf-option (and at the same time does not put out an error message).
Could perhaps someone confirm this?
It would be also very helpful, if a savvy person could exactly explain, what options are supported and which ones not.
Happy to discuss.