If im planning on getting the 3600 and dont intend to upgrade to 4000. Is there any reason why I should wait out for B550? Or would B450 be plenty?
i wouldnt recommend it unless you require pcie4.0. i just bought a 3700x and b450 tomahawk max yesterday

A520 is most likely to replace B450 in the upcoming months, while production for B450 will continue til end of the year. This might not be accurate til further news leak
This means B450 price will not drop much as there are production ongoing and B550 will be a "more" premium board if u want extra m.2 or wifi6+bt
I am not sure A520 is going to replace B450 chipset. It sounds like it should be the replacement of A320 which is overclocking locked. In fact, it may be possible that A520 is a rebrand of B450 with AMD locking the overclocking feature.