You can definitely ouput only the sound to the avr by optical and the picture direct to the tv via hdmi. But probably this only gives you a single source without moving cables around. A hdmi switcher with separate audio out (make sure it has hdcp 2.2 for 4k picture) is available on amazon for under $100. Run its hdmi out to tv and sound to avr. Then you can have xbox, ps4, cable box all work without moving any cables.
If your receiver has earc then you can get the sound from tv to receiver via hdmi.
Upgrading from 5.1.2 to 7.1.2 is a trivial change. The usual upgrade path in order of importance is 5.1 -> 5.2 -> 5.2.2 -> 5.2.4 -> 7.2.4. A quality sub makes all the difference, make sure it is from “the list” on subwoofer101.com or will just give you a headache. The good subs are not noticeable most of the time, blend in subtly, then rock your world with fidelity when needed. Was a gamechanger for my theater previously with expensive 10 year old subs that provided one note headache inducing bass.