Does a flagship lens show up in the finished photo, or only at 200% on a monitor? Lenses

Sharpness is the thing people compare and it is the thing that matters least at your output sizes. What actually shows up in delivered portrait work is the rendering of the out-of-focus area, the shape of specular highlights toward the frame edges, and how the lens behaves against a bright window, plus autofocus that keeps up when someone turns their head. Your complaint is literally the busy background, which is the one area where the expensive glass genuinely earns the money. That is a real reason to upgrade, unlike wanting more resolution you will never print.

Before spending three times the price, try moving. Busy background at a portrait distance is usually a subject-to-background separation problem rather than an optical one, and two steps forward with your subject and ten steps of background behind them does more for the look than any glass. Then if you still want the lens, at least you know it is the lens.

Rent it for a weekend and shoot a real session, not a test. Two hundred percent crops on a monitor answer a question nobody has ever asked a photographer.

Owned both in one focal length for about a year and sold the expensive one. For me the difference at A3 was visible if I looked for it and invisible if I did not, and the weight meant I left it at home for anything casual. A lens you do not carry is worth nothing. That is a personal conclusion, not a general one, and someone shooting the same subject in worse light would reasonably decide the other way.

Also look at the used market for the previous generation of the flagship. The optical formula often barely changed, the focus motor is a generation behind, and the price gap between it and the current one is frequently larger than the difference in the pictures.