there is no way to get 5.1, since the HomePods only do singles or stereo pairs
It will play atmos or 5.1 in a simulated way, using a HomePod (single or pair, and full-size only, no minis)
you need to set "default audio output" in the settings on the aTV for this.
Use HomePod with your Apple TV to create an immersive theater experience.
support.apple.com
You do need decent wifi to do this, If your router is on one side of the house, and the tv room is on the other, you might run into issues, If you can hardwire the appleTV to ethernet you might have better results.
also this will not connect to audio coming from you TV, so if you have a cable box, game system, or use the apps on the TV, then you'll be stuck with the TV speakers for those sources.
I'm a huge fan of sonos.
if you get the beam or Arc, they connect to your TV by HDMI, no worries about wifi strength. also makes lip sync much more reliable.
You do need an HDMI-ARC port on the TV to make this work the easiest, if your TV is under 5-10 years old and not the very cheapest model it probably has one. (look at the HDMI ports, one of them should say ARC, normally #2)
You can add a stereo pair to either one as surrounds, and also 1 or 2 subs, although those are pricy.
once the TV audio is in the soundbar, you can play it on all other sonos speakers in your system. (although I think those are limited to mono/stereo)
all of the new speakers also support airplay, so any audio on your phone/Mac can be sent over the network.
has support for playing a lot more streaming services directly on the speaker. this is a better way to do it, airplay can be a little flaky, and you don't have to worry about a video in some random page you visit on the phone taking over your big speakers. HomePod currently supports apple & pandora, others supposedly coming. sonos has apple, Spotify, pandora, and tons of others (their site lists around 100, but probably less depending on your country)
and since it's playing on the speaker, since their apps are just remote control, they're available for everything. If you have android or windows people, sonos is much friendlier to them.
If you're airplaying to Hompod from your phone, you have zero control from your Mac
If you're airplaying to sonos from your phone, you have basic controls on your Mac (play/pause, skip forward/back, and volume)
you do loose siri, but gain Alexa and google assitant, Siri is good for integrating with apple things - messages, calendar, reminders. And she shines at home control with homekit. Alexa/google seem better at general knowledge questions - "what time does X open" plus Alexa has control for a lot of other devices that aren't available in homekit, like robo-vacuums as it's pretty easy for other people to code skills for Alexa to allow her to do a lot.
Ikea has sonos compatible speakers. They're fine for surrounds, and probably more than enough in an office or similar where you just want background, and not full volume.
Prices are US dollar
for comparison -
hp Mini - $99
hp Full size - $299
ikea symfonisk - $99
ikea "lamp speaker" - $189
sonos one - their base speaker - $179/$199 - depending on if you want Alexa or not
sonos beam - $399
sonos arc - $799
sonos does occasionally run sales, and you can normally save a few bucks if you buy sets.
