After a quick read through this thread, I would second many opinions to ditch going to BestBuy as your only place to purchase speakers (TVs and audio components are one thing). I'd also return the soundbar for a more traditional set up. Lastly, if you hardly play any video games... there are better 4K players with streaming that will suit your needs for far less.
For starters, I'd get this receiver from A4L. A lot of folks have purchased from them, myself included, great customer service and buying AVR's brand new has never made much sense to me. Take the savings here.
^ This will do your 4K pass through, meet all the current HDCP and HDMI standards... Atmos, DTS:X, etc etc.. If you think you'd want to play louder, maybe spring the extra $30 and get the S920W. After shipping your at $270 here.
If you really wont play any video games, I'd return the Xbox and grab one of these 4K players. I've got the same one and have really enjoyed it to date:
^ that'll set you back $200 (but would end up being a wash with the return of the Xbox). Has the streaming apps (though your TV does too) but I feel like it has great picture quality for all kinds of movies (4K, Bluray, regular).
I'd then personally suggest you get a pair of Chane A1.4 bookshelf speakers for L/R and the Chane A2.4 center. The center, as you know now, is by the far the most important piece in a HT set up. Many folks here, including myself, have found it to be really robust. You'll also get plenty of low end out of this front 3 until you can save up and afford to get yourself a sub.
^For the pair of speakers and center, it would run you $569 plus shipping. I'd also recommend dropping a line to Chane's email and reach out to the owner
as perhaps he has some B-stock or a shipping deal going on.
So if you return the Xbox and get the 4K player, you'd pocket yourself a savings of about $50. The AVR and speaker combo I've recommend would come in at $839. Subtract the $50 savings from the Xbox return and your at $789. Slightly above your initial $750 budget, but I think you'd find it a really great system.
I'd also recommend reading threads like this one here to give yourself an idea of other speakers and their performance. This thread in particular is a wealth of knowledge and good contributors for budget speakers.
Happy hunting and good luck-