I fear you are expecting too much subtlety from these pens or any make of fine liner/brush pen combination.
A "Unipin" brush/pen will go as close as it is possible to get the effects you are trying to achieve but only using a smooth Bristol
a vellum Bristol board will seriously hamper your objectives here regardless of pen type.
Ultimately though, I think the use of traditional pens & ink is what will get you to where you want to go.
Another possible route might be to use marker paper but that is not suitable for any original work of course.
PS. I've just noticed you are using a different make of pen & brush
That is the very place to start ! use the same ink,
black ink has more variation in it than ANY other colour to say nothing of different manufacturers chemical mixes.