You can always keep an eye open for deals on a 4K receiver. Denon regularly has closeouts, but I'd stay away from the bottom of the line models. Step up a model or two at least.
Apps in smart TVs do tend to be crap. Streaming devices are much better, but you need to be able to connect them to the receiver, or you end up with degraded audio.
Because most TVs output 2.0 through optical from sources connected to their hdmi inputs. Just the way it generally works, though there are some rare exceptions.