Unless you really want Google Assistant, you can get soundbars that have Chromecast built-in making the 3.5 mm AUX jack inferior to getting a proper soundbar. I'm sure using the AUX jack would be an improvement over most TV speakers, but the digital sound processing in soundbars should provide a better listening experience. And don't forget, if you want Google Assistant, you could get a Google Home Mini and tell it to cast to the soundbar.
And while you could get a soundbar without Chromecast built-in and just add a Chromecast Audio to it, it sounds like you're trying to simplify your setup, so I would definitely go for a soundbar with Chromecast. If you already have a soundbar and want casting capabilities, you might want to first try adding a Chromecast Audio to it before buying a Google Home Max.