I'm not sure it's helpful to take the internet advice (or even a teacher's advice) as a hard rule with no exceptions. The world is not as black-and-white as that. The advice that you do not need a solid silver head-joint until you are grade X, or that you do not need to upgrade your instrument until it is holding you back, was probably given in the context of cash-strapped parents or anxious players, who'd rather not be buying another instrument, and need reassurance that they can still get plenty of mileage from the instrument they have. It doesn't mean you must not buy better. It just means you are not obliged to buy better now: you can put it off for a year or two if you want.
There are also plenty of people who enjoy owning and playing a nice instrument, and there is absolutely nothing wrong with that. The recorder world is full of people who enjoy playing a beautiful wooden instrument, even though they'd be the first to admit that technically they're nowhere near the level where they can extract from it all of its beauty. But I think many would be upset if a shop refused to sell them anything better than a £30 Yamaha plastic on the grounds that the plastic recorder is quite adequate for their grade. We have the right to buy a nice instrument if we've got the cash in our pocket. And it rarely does any harm to have an instrument that's a bit too good.
So if you want a better flute and can afford it, go for it. Shops will advise on what will make your flute better. They will probably advise (if asked) on the suitability of an instrument for a particular person's physique and intended use, but they'll assume you know what you're doing, and they won't intrude in your personal decision about when to upgrade. They aren't there to be judgemental.