There are a lot of AVRs that have bluetooth connectivity that are compatible with Alexa, like
this Yamaha. Any of the mid to upper tier Yamaha, Denon, and Marantz receivers can do that. Onkyo as well.
As for a center channel, they can be advantageous, especially when listening at off axis angles where you are not equidistant between the speakers. If you only ever listen in the sweet spot though, you might be able to get by without a center speaker without noticing much difference.
As for surround speakers, you might as well get wired ones. Even bluetooth speakers have to be plugged in, so they need to be wired one way or the other. If you don't need anything fancy, just get some
Polk T15s. They have a keyhole mount on the back and are intended to be wall-mounted at a higher position than level with listening position, which is good for a surround speaker.