RoystoysOh, there are many great or brilliant flagship alternatives. Oneplus, of course, but also Oppo, Xiaomi, Sony, Vivo or (for much less, if you accept less than flagship CPUs) Poco and again Xiaomi. Just not alternatives with a comparably small format. Basically, if size is key (as it is for me), just Samsung S21 for absolute flagships; and, if older (sub)flagships are ok, Samsung S20/10/10e/Note10 and Pixel 5/4a/4. And, perhaps (but it's far too tall for me), Sony 5.
If your going to spend that much you can't go past the OnePlus 9 Pro three years of OS and four years of google security updates. Even if you don't keep your phone that long,and let's face it who keeps a phone for more than a year if they can afford it,it is always easier to sell for a decent price if it has OS and security updates coming. It is also one of the only phones that support Miracast,if that's something you need like me