I used to run CyanogenMod OS on Galaxy devices. Now I'm on a OnePlus device running their OxygenOS with root via Magisk and haven't felt the need to do much else with it. I have ad blocking, firewall, etc. and I generally use Brave browser with settings tweaked for maximum security & privacy. I would be very careful and hesitant buying used devices from the Internet but maybe I'm just paranoid.
Anyway I'm not quite ready for an upgrade just yet as my current 6+ year old device still does everything I need it to do competently, the battery is solid, it's fast; but I'm curious about new OnePlus and Xiaomi devices. I probably won't buy another Korean device ever again though. I had a first gen Samsung Galaxy S (the very first one in the series) which was pretty good at the time, and I've had numerous other Galaxy devices over the years, but they've gotten so bloated and are among, if not the, worst when it comes to unlocking and rooting, and are ridiculously expensive, so my next smart phone will likely be Chinese again.
My current OnePlus device has been the best performing and least problematic smart phone I've ever owned and I'd have to go back to the old Nokia brick phones to think of something that was as robust, and probably Motorola Razr a close second, but neither were really "smart phones" so apples and oranges. The Chinese phones have really come a long way and imho are now ahead of everyone else in mobile technology, performance, reliability, etc. Compare the latest Xiaomi and One Plus vs iPhone, for example. Although I suppose the SnapDragon/Adreno chips they are using are American (Qualcomm), but built in China (TSMC).