Microstrip linear taper effectiveness
Notice this key paragraph in the Microwaves101 article you cited:

A taper is a high pass structure and will work well at all higher frequencies. The limit on the low end turns out to be how long the taper is compared to a wavelength. From figure 2 you can see that for the taper to work to a -20 dB level (20 dB reduction in maximum reflection coefficient) the length needs to be roughly half wave length at the lowest frequency. If you want to work down to 2 GHz the length has to be half wave at 2 GHz. If you want the taper to work down to 1 GHz it has to be twice as long.

And the accompanying graph:

[source]
In your case, your wavelength is about 35 mm (for 6 GHz with velocity factor 0.7), and the length of your taper is about 0.5 mm, so you're well on the left side of the graph, where the taper provides no advantage. Even at 18 GHz, the wavelength is about 12 mm, and your taper length is still below \$\lambda/20\$, so you should not expect any advantage.