4-6 months is very optimistic from Poorboys Wheel Sealant though - it's okay, but won't last anywhere near that long.
Treat your wheels like you would your paintwork - they are just painted and clearcoated just the same, and are as delicate, but are obviously subjected to far more **** than the rest of the car.
One of the dedicated metal sealants like Blackfire Metal Sealant or Collinite 850 (a gem often missed!) is a great choice for wheels, and can also be used on a freshly polished exhaust. They leave a superb finish, and are designed to withstand the high temps, so are ideal for brake dust etc.
I find most of the dedicated 'wheel sealants' a complete waste of time - you're often better off just using a normal wax. Sealants work well too, so EGP for example, or Zaino Z2 or Duragloss 105.
With regards to Bilberry (or any wheel cleaner), yes they will degrade the levels of protection over time.
Suggest you pressure wash your wheels first to remove the majority of the brake dust, then hand wash them with a dedicated mitt and bucket and a decent pH neutral shampoo. Dodo Juice have a dedicated wheel mitt (Captain Crevice) which is very effective on the Clio's multispoke wheels.