I agree. I had once a wifi card in my x86 router. It performed well indeed, but:
it was setup to broadcast on 5GHz band and sometimes I needed the 2.4GHz. Can't do both at the times.
the wifi card worked 24/7, and so dissipated heat (very).
Finally I removed it and I use a recycled router as an AP instead. Performances are identical, I have both 2.4 and 5 GHz bands available. Heat is much lower on the x86 router. I can poweroff the AP when unnecessary. So a separate AP is a better choice.
if you want take it really easy, look for something like that. you can flash cudy vendor openwrt first, and than openwrt. simple process, without puzzle, especially if you are new to openwrt. logically there are many devices easy to install openwrt, this is just an example.
This is completely wrong, if the chipset supports, it can run 2.4GHz & 5GHz at the same time. For example I have this card, I can create both 2.4G/5G WiFi at the same timie, in fact most router hardware using single chipset for both 2.4G/5G.