The name is Ken. Not Simon.

The quote from the article feels like "oh hey, he mentioned an 85 and I do have an 85, too, so this must be totally the same situation...", but it's not.

If you are looking for

non-studio places like [...] a graffiti wall in a street

as backgrounds, will a more tele lens be helpful? I doubt it.

Basically speaking, the longer the focal length, the less background you get in your image. Using a 300mm lens defeats the point of going out and finding these non-studio locations to a certain degree, because you are showing substantially less of them in the image.

After all, you are making a simple studio shot more complicated, by replacing what could be a small colorful backdrop with a giant wall in some street of which the lighting is probably not under your control.

If non-studio is really the goal here, then you do not want to simply fill the area around the head with some arbitrary colorful blur. You want to fill this area with a distinct background. The background is in a way a subject of its own. Not the main one though.

If these portraits are not staged, it gets even worse. The 70-300mm is twice as big as the 85mm and twice as heavy (roughly). You will be recognized as a photographer more easily. A "portrait" made with a 300mm or other very long lens will show a distance to the subject. It's not like you get that intimate moment at the other end of the street with the longer lens, because you are in fact at the other end of the street which reduces the intimacy of images dramatically, you wouldn't be close enough, and if you're not close enough, you will remember how that quote starts.

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tl;dr

• non-studio = wider/normal/slight tele focal length, not longer; because background/surroundings matter
• your stuff is awesome for what you want to do
• spend the $1000 to find even more interesting non-studio places, hire assistants, models, etc. instead of buying gear
• if you still feel like you want the 70-300mm, rent it