Huawei has a custom OS but it does most of the stuff including all health/sleep tracking features and music playback. Sure, their app ecosystem sucks but honestly, even Wear OS is pitiable(when compared to something mature like Watch OS).
Silicon Carbon battery, IIRC, can pack higher energy densities than comparative Li-on batteries, though it is not a very significant difference as of now.
Upto 6000mAh battery is common. Some Oneplus phones have upto 5500 mAh, Samsung budget series have an option upto 6000mAh. And of course, Chinese OEMs do offer tons of phones in that range.
Now, what is interesting is that phones above 6000mAh are rare. Whilst there is the occasional power bank attached phone that weighs half a kilo and has five digits battery life; the most I have seen is a Techno phone going upto 7000mAh.